Rear end feathers off on Buff Orp hen...why?

[stroller]

My year old Buff Orp hen Scarlet is now most aptly named. ALL the feathers on her back end are missing. Just starting to grow back now, but her butt looks quite red. Any ideas? She's in a small flock with a banty rooster, four other Buffs, an Americauna and a Belgian d'Uccle hens.

 

[eggfire]

I just recently read on here about a buff that had a mite infestation causing her rear end to turn very red and swell up. The thread was started by alaskapalo, I believe. You may want to check it out.