Sounds like other birds may be doing it. Do you have a rooster? I've seen hens peck on each other's head pretty badly to try to get other hens out of nestboxes so they can use them.
Another time will be at roost time. There can be lots of pecking and jostling for a good position especially if you don't have a lot of extra roost space.
As long as there's no blood or wounds it shouldn't be a problem, mostly cosmetic. If you start to see blood than you may have to try to figure out what's going on.
Sometimes it can be from crowding or boredom. How much room do your birds have?
Chickens will lose feathers here and there as the season wears on. Most won't grow them back until the fall molt. You can spray the skin with bluekote, or an anti pick spray. Be care you don't get any in the eyes.