My buff orp has the same problem, but hers is because she's so fluffy back there the poo collects on her feathers and pulls them out. My daughter and I have to do "poop-ectomies" every once in a while. May not be related, but she doesn't seem to have laid an egg in the 13 months we've had her.
My Buff Orpington has something similar, but her eggs have always been normal. Poop collects on her butt, and this protuberance seems to be the cause. She acts perfectly normal - eats and drinks fine, lays a normal egg almost every day. Other chickens don't pick at her. I can't figure out what the problem is, either.