Why did my rabbit bite my nipple?!?

I was sitting on my couch holding him and brushing him. Normally when I groom him he will lick me and lightly nibble back as if he were grooming me too, and he was, until he bit my freaking nipple! Hard! And then, as I not satisfied he'd done enough damage, he did it again! He even put a hole in my top! Obviously I'll wear a bra from now on while holding my rabbits but I was in my PJ's! I don't understand why he would do this... He neutered and has never shown any sign of aggression. I know rabbits will nibble at the knots in each other's fur to loosen them, is it possible he thought my nipple was a "knot in my fur"? I'm sorry I'm not trying to be vulgar but I have to know why he did it... Twice!