Sudden humping from bonded Rabbits

I have a male and female rabbit that have been in the same cage since birth which was about three years ago. The male has been fixed for over a year. They have always been very affectionate to each other until recently the male has been constantly trying to hump the female. She hides in the house with her head sticking out. As soon as she comes out, he jumps on her. This has been going on for about three weeks. It doesn't seem to be letting up. Thoughts?

If she was never spayed, that could be playing a part. Bonds with an intact rabbit have a higher incidence of breaking suddenly for no apparent reason. He may be sensing her hormones or moodiness.
Another possibility is that one rabbit may not be feeling well. If one rabbit is grumpy, that could also cause sudden aggression.