Chickens peck at red, as in red skin or bloody skin and feathers. They develop a habit and it is contagious among the flock as damage increases.
Some solutions other than dietary, as mentioned above are to make sure that roosting space is more than otherwise thought to be plenty. Then isolate the defeathered hens and isolate the aggressive hens. If necessary to break the habit, the tip of the upper beak can be shortened just enough to disable the ability to produce the sharp pinch.
