In my m/f pairs I have also seen the males hump the females occasionally just for fun. The males are the submissive partner (based on grooming patterns) and the females don't resist, so I'm guessing it is more for nookie purposes even with everybun fixed.
I love how the same behavior can mean different things in a different context. Rabbits are wonderfully complex animals.
In your case it will probably go away as they settle in.