That's weird. It is around the sort of area a doe will pluck when making a nest.
Does the rabbit live indoors or come indoors? I'm thinking allergy to something here, my old dog had something similar, she was allergic to lots of things and plucked fur out when she itched. Had to change lots of things, including our washing powder.
I'm assuming fleas have been ruled out?
Do they know whether the rabbit is pulling it out, or whether it is falling out?
Over what period of time did it appear?