Apparently, Berry made a bet with Pinkie Pie on who could throw the best party. Of course, Berry being Berry, she thinks that the best way to do this is with a little alcohol. Nothing too heavy. Just some cider with enough booze in it to get a pony buzzed. But she never guessed that Pinkie Pie is a light-weight. So light-weight in fact that within a few drinks, she's already tipsy.