Check her for lice and mites.
The others are picking her bottom clean and it looks like there's a scab, hard to tell by the blurry photos.
How much space do they have? Picking can be from boredom, lack of space, they drew blood somehow and now want more, nutritional deficiency...
Cover up her bare bottom with NuStock, this can help deter the picking a little and help heal the skin.
Is she laying eggs? A plucked bum can look full or bloated just because it's bare or if she is fluid filled or bloated, then she may be at the beginning stages of a reproductive disorder. It could be just fat, which is also not a very good thing.
Has there been any changes within the flock, losses/additions, change in feed, bedding, location, etc.?
