does mount other rabbits as a dominance thing. i've had does mount bucks when they were really ready to mate - it initiates the runaround where she wants him to chase and catch her. i typically don't let the does mount the buck too much, especially if he's young, because apparently they can make him lose confidence. i suggest checking for sure if she's a doe, but it is fairly normal for it to happen. i would separate them for sure if they are housed together. it could continue to get more violent - does are very territorial.