This is quite the mystery. Have you checked to see that her oil gland isn't inflamed or infected somehow? It's located in that area, and if it was irritated, I could see her constantly plucking and bothering the area.
The only other idea that I have is to put pinless peepers on the whole flock. That way you know for sure they arn't plucking her, and that SHE isn't plucking herself. Plus they look hysterical.