Applejack knew that Apple Bloom was growing up, and she knew that one day it would be her job to tell her about where baby ponies come from, why she suddenly has weird feelings for colts (or fillies, maybe), and what happens to mares every summer that brings the previous two facts full circle. She just didn't think that time would be now. And she also didn't think she'd have to tell her sister's two best friends about it at the same time.

In hindsight, Rarity probably wasn't the best pony to run to for help.