My female rabbit suddenly biting and mounting my male?

[morganforte]

I've had my two rabbits Willie and Sadie for about a year now. They are both fixed and have been fixed since they were 6 months old (they are both almost exactly a year old now). They were not introduced to each other until they were both fixed. They bonded quickly and easily with no aggression issues and were the best of friends for nearly 6 months after the bond-up until about a week ago. My female rabbit, Sadie, who is smaller than Willie, but definitely more spunky, has started biting Willies hind quarters and mounting him. She also chases and nips him. It really bothers me and all Willie does is run away. He never tries to fight back (which I suppose is good because I don't want them getting into actual fights). She's never drawn blood from him but it still makes me nervous. They also seem to be getting very lazy with their litter box in their playpen, but my main concern is the sudden change of temperament from my female. I just don't want Willie getting hurt! She seems to do it more when I am petting Willie and not her. I think she gets a little jealous.
Do you guys know what the cause of all this is? Or possibly how I can fix it? Thanks everyone!

 

[doodlebugger]

If they are both fixed, it just sounds like a dominance thing to me. I am not sure if she could be experiencing some kind of hormonal issues if she is fixed or not.....that might be something to check on. I have heard of rabbits who were bonded and at some point had to be separated because of fighting, but it was due to the female being hormonal (she was not fixed, and even after she was fixed, it didn't help). I would be a bit concerned as well....would hate for one of them to get hurt if they are fighting. Hopefully someone else will have more sound advice for you.