It looks like another chicken is picking out her feathers. Are there others who are missing feathers or one that looks perfect? Can you compare her bottom to other hens to see if hers is really enlarged? They may pick feathers if they are lacking enough protein in the diet, if they are overcrowded, or if bored. What are you feeding her? Are they getting out to roam in the yard? Do they have enough room in their coop, on the roosts, and enough nest boxes?
I agrer it looks more like she's getting plucked and pecked. Generally it's from a protein deficiency or from crowding and boredom. I would spend some time quietly observing your flock to see if you can see when it's happening.