Sorry for your loss. Chickens may pick on each other picking feathers and vent pecking for various reasons. Those can include being cooped up without getting to free range, too little room or boredom, lack of protein or feeding an unbalanced diet, too much light in nesting areas, and some breeds are just more aggressive. It can lead to cannibalism. If you have any pictures of the feather loss, we can help. Do you have a rooster? What are you feeding? Do they get to free range or get out in the yard? Could they have kept her from drinking or eating?