She's getting chewed on by the other one, probably. Every once in awhile you get one that does that, and there's not really a reason for it. I had one such last year; chewed on everybody for no reason other than she though it was fun. None of the others did any chewing.
Eventually they grow out of it. In the meantime, I find that offering other fun things to chew on like toys, and providing a lot of grazing time can help.