From the photos it looks like Britt is talking about the back of her head, not her back or sides. When a mating takes place the rooster grabs the back of the hen's head. That's the signal for her to raise her tail out of the way to expose the target. Watch sometimes and you'll see what I'm talking about.
I don't know why her feathers are coming out and the other hens' feathers aren't. I also don't know how to stop it. I'd certainly keep an eye on it. The risk is that when he grabs her he cuts her with his beak. I don't think blucote or something like that would help since it is part of the mating act.