Simple--bunnies don't like to be touched on the mouth. Some bunnies are "hand shy" meaning they don't like to be touched by hands, frequently because they've been mistreated in the past. My Muffin came to me like that. She was in a bad situation before she got to the shelter I adopted her from--neglect/abuse. After a long time of only approaching her slowly with a hand, and frequently having a treat in that hand, she started to let me come up to her and pet her (took months). Another thing I did was go slowly up to her with my hand, and if she jumped away, follow her, and pet her in a way I know she liked (base of the ears)--that way she couldn't really get away from being petted, and was able to learn that it was a good thing.