I would imagine it has something to do with their wild kin digging in the dirt all the time, you don't want that going in your mouth if you're a bunny. Dogs don't care, but....
I like the sound it makes...a speedy 'whup-whup-whup' :lol:
It's almost as if they're saying, "Tch! I'm SO filthy...MUST have a wash!"
Because, you know, bunnies are such grubby things...hahaha...right, mine are cleaner than my cats! :roll: