I have three young hens that just started laying, two of which are red stars. I had noticed in the last week or so that sometimes I would see one of the red stars pick at the other's back, just before the tail. Today I saw that the picked-on red star has a big bald patch in the vicinity of where the other had been picking at her. I watched them for about ten minutes and witnessed the bully continue to pick at the feathers around the bald patch on the victim. In my case, at least, it's clearly a matter of one (or more) chicken(s) pulling feathers out of a flockmate, and I'll probably need to quarantine the aggressor(s) for a while. In your case it might be a more mundane reason, as suggested by mshaffer. Have you taken a good look at your chickens to see if anyone has bald or bleeding areas?