I can't remember enough about this and I'm stilla rabbit beginner but I do know it's nothing to worry about too much.My rabbits get it on and off too and I believe it's something to dowith them excreting excess calcium, (in my rabbits cases I'm guessingfrom when they've been naughtily chewing on the plasterboard walls,something I just can't break them of doing )
Hopefully an expert out there will be able to give a betterdescription of the hows and whys of it, I just wanted to let you knownot to worry .