I tend to agree with the above 2 posts, maybe your bunny's teeth are too long?

While I don't have any experience with bunnies, I did own guinea pigs for 10 years and I know they both have the same type of teeth. The teeth keep growing and they have to keep them worn down by chewing on things (like wood). If they are too long a vet should be able to clip or file them down or something, I'm not sure on that as my guinea pigs never had that particular problem because they always had lots of things to chew on.