Blame the alcohol

Twilight slowly stirs, her plot sore "Owwww... where am I?"

Observing her odorous, plastic environment, she surmises that she had passed out using one of the Canterlot world fair's numerous port-a-crappers "Ugh... I can't smell anything but rotting dung... gotta get out of here..."

Pushing the door open, she step out onto air, falling into a heap of gravel "... Who moved the crapper?!"

"Twilight?"

She slowly raises her head to regard the owner of the voice. Rainbow Dash.

"Hello Rainbow... why is there no sidewalk outside the crapper?"

Rainbow looks around her, sucking her bottom lip "Aaahh... you see... that is..."

"Spit it out please" Twilight says, brushing gravel out of her mane and wings.

"You're... sitting in it. Well, what's left of it."

Twilight gives Rainbow a flat look "What are you talking about? How is some gravel an entire sidewalk?"

"Look around you Twi."

Twilight takes flight and her stomach drops "What... what happened?"

Rainbow shrugs "Beats me, I was buying a wing massager when half the fair goes 'Puhbthblblthr pth pth...' like somepony flushed the world's biggest toilet and it got backed up by a giant load!"

Wiping the spit from her face after Rainbow's onomatopoetic, Twilight rolls her eyes "The world's largest toilet? I'm pretty sure that-"

Rainbow shakes her head "I know the sound of a giant toilet being flushed, Twilight. And that's what I heard."

"Why would you know what that sounds like?"

"I work with weather. The whole raincloud machine? It's really a giant upside-down toilet. I'm serious, that's what it was originally, a giant toilet."

Twilight files that bit of information in the back of her head "Okay... so why would a giant toilet back up?"

"Because somepony took an even gianter crap! Keep up Twilight, it's embarrassing."

"One, I am keeping up. Two, 'Gianter' Isn't a word."

Rainbow sticks her tongue out "Says you Purple Smart."

Twilight sighs "How come I wasn't affected by the-"

"Demolition dump? Gargantuan Gross? Catastrophic crap? Disastrous deuce? Supreme shit? The big one? The hail sewage? Smooze surge? State of emergency bowel evacuation? Celestial clogger? Prolapsent movement? Water birth?" Rainbow lists

Twilight face hoofs "Yes, that."

"I dunno... princess perks?"

Twilight rolls her eyes "I don't think so. Where are the others?"

"Fluttershy ran away and Rarity's looking for her, Pinkie is keeping ponies entertained, AJ is rescuing ponies from the wreckage, and Spike's digging tunnels for the ponies trapped in the underground sections of the fair that collapsed. I was sent to look for you. And you were napping in the single standing crapper in the center of the crater."

Twilight flies higher and the blood drains from her face.

Sitting in the exact center of the blast zone, a single, solitary port-a-crapper stood as if it was the hole in the center of one of her mother's fruitcakes that Shining and Cadence always chip their teeth on every hearth's warming's.

"I... if it had been even a hoof off... I would be dead... a princess flavored flambé..."

A slick, warm something crawls up the side of Twilight's face "Meh, you taste like sweat and stale urine... and that funny smell of dirt after it rains... Petrify or something"

Twilight turns to see Rainbow smacking her lips "Did- why did you just lick me?"

"I wanted to know if princess flavored whatchamacallit tastes good." Rainbow replies, as if it were obvious.

Giving up on finding reason in this, Twilight returns to the ground, looking around the area of the crater "What do you think happened and don't say 'Shit'."

Rainbow bites her lips as if holding something in "... Shit happed... Bahahahahahahahaha! Haahaha!! Sh-shit happened!!"

Groaning in annoyance, Twilight inspects the rectangular pillar that still held the stinky prison atop it "Why would my single port-a-crapper be spared?"

Loud wing beats echo off the giant crater, Twilight turning around to regard the source "Pr- Celestia!"

Celestia grins and takes a quick look around the crater "... What happened?"

Rainbow sits up and opens her mouth when Twilight hisses at her.

Rainbow rolls her eyes "Shit happened, a whole lot of shit happened."

Celestia blinks "What?"

Twilight sighs "Rainbow is under the impression that a giant toilet got clogged by a massive shit and exploded from the sound she heard."

Celestia blinks and sniffs the air "Twilight... you haven't been taking any... stool softeners have you?"

Twilight nods "Yeah... I started today because of all the greasy food Pinkie was going to shove down my throat, why?"

Rainbow snorts "A hard plot like you? I'm surprised you haven't since birth."

Celestia sighs "Did you read the label?"

Twilight shakes her head "No, why? I figured that a table spoon was enough."

Celestia groans "The one time you don't read something... Twilight, what brand was it?"

"Uhhh... 'Bowel fairy deluxe, supreme'?"

Celestia sighs "Twilight... allow me to show you what happened in the form of a magical simulation..."

Twilight crossed her legs, her insides feeling like they were revolting against the confined space that was Twilight's skin "Urrg... gotta make it to the... ooohhgg..."

I don't rememb- where did you find the popcorn Rainbow?

Brought it with me.

After weaving through masses of ponies to a single, solitary port-a-crapper "Oh thank my beard!"

I don't have a beard!

Shush, dis gon be gud...

After depositing her slowly enlarging royal posterior, she let out a base moan in relief "Aaaughblrgl bmmmngrgh..."

Why do I sound like some inebriated mental patient?!

After sitting for a good few seconds, the revolt hand been quelled by her stillness so in order to allow her to not soil herself like at her tenth birthday party-

You promised you wouldn't tell!

She pressed a hoof to her swollen gut, a horrible idea in hindsight. But nothing came out, for the turtle in her sphincter lay dormant in the stillness. So to avoid splattering everypony should she try and stand, she rocked side to side, the revolt sparking anew with a groan reserved for the bedroom for most, but the bathroom and shower for our dear purple princess.

Wait... that voice... DISCORD?!

There be nochicky here but us Scootaloos... bawk bawk bawk...

Sounds like Squirt to me.

So forceful and intense was her moan that for an instant, Rarity's coat coal black by the ambient wave of perverseness, Pinkie accidentally inhaled chili pepper dust, Rainbow's mane lost color-

I can attest to that fact.

Applejack sat on an anthill that wasn't there before, and dear, sweet Fluttershy grew stallion parts that frightened small fillies.

Why would she grow stallion parts?

I'm pretty sure we were just told why.

It was so chaotic but so perverse that the humble, handsomely modest and ladies' man lord of chaos retched a dry heave of babies and puppy kisses over Princess Celestia, who shriveled up under such intense purity, her very soul absolved of the sins she hath committed against her own kind and the aforementioned hunk whom she prostrated herself before, begging forgiveness for her past actions and implored his magnificence to let her bear his divine children.

That's definitely Discord.

Noshrubby here but us bushes... rustle rustle...

Back in her cheap, plastic throne room, Twilight groaned and strained to push her ugly, deformed love child from a saucy romance with a hypnotized Spike out and into-

I haven't even had- you know what, no. I'm not feeding the troll.

Dayum Twi, if I was you I'd wipe real hard before you go out... or I could hook you up...

Shut up Rainbow, Shut up Discord.

Noling here but us changelings... chirp chirp chirp chirp...

Finally, with one last inequine heave, the head of her burden breathed its first breath. Twilight sighed, her horn sparking in concentration to pull it out of her straining body.

A passing, innocent mail mare knocked on the door having heard the groans and offered to get help, but the gluttonous Princess denied it after another inch of her insides spasmed and with the most powerful surge of magic fueled by her having been full of crap from birth, the massive log up her arse made way for the freedom run of all of the evil and malice within her bloated body.

THAT'S IT! I'M GOING TO STOMP YOU FLAT DISCORD! I WILL USE YOUR DUMB GOATEE TO FLOSS MY DOCK!

Nobrony here but us cloppers... bawk bawk bawk...

So massive was this single turd that when it hit the water, the resulting ripples devastated half of the fair, leaving a huge crater and in the center of it all... the stewing nursery for the cataclysmic dump... Princess Toilet Sprinkle...

The holographic light show disperses, leaving Twilight angry and Rainbow floored in laughter "It didn't happen like that."

Celestia shakes her head "No, that is what happened. I was told by the sole witness of it all."

Stepping aside, a happy looking grey pegasus with a cookie in her hooves smiles at Twilight "Hiya princess Twilight!"

Twilight rubs her head "Derpy... why did you make all that up?"

Derpy shakes her head "I didn't. I heard you go plop and then the ground was gone!"

Celestia nods "Yes, I just filled in the few blanks left by Miss Hooves' testimony. For the remainder of the year, you must drink so much milk that your system backs up to normal conditions. And stop molesting others in their sleep. It makes it hard for my sister to 'Mind fuck' them. Good night Twilight."

Taking Flight, Celestia returns to the catle, preparing to address her subjects on what had happened.

Derpy waved and took off as well, most likely in search for food that was going to spoil without ponies to eat them.

Twilight Sparkle's eye twitched "Wh- what just happened?"

Rainbow tosses her empty popcorn bag in a wastebasket teetering on the edge of the crater "You took the biggest dump in history, destroyed half of the fair, insulted Discord, and rid yourself of all the shit you were full of... and I got me a wing massager and twenty bottles of 100 year zap cider. I don't know about you Twi, but I'm going to the castle, pop open one of these babies and taste the rainbow~ And myself, because being a pegasus means you can lick your back and more! Wanna blow your mind instead of your bowels?"

Twilight nods "I... I need something hard yes..."

Rainbow grins "You're gonna need something else that's hard..."