Assuming you’re in the northern hemisphere, sometimes a change of seasons (particularly into spring) can bring on hormones associated with that, regardless if bunnies are neutered or not which can lead to some territorial mounting etc. Humping the head isn’t uncommon, but it can be a little more dangerous for the instigating bunny who is more easily injured if the bunny underneath decides they don’t like it and wants to bite.

Even my bonded pair who have been bonded for several years, Luna will sometimes mount Apollo, head or tail. Not every night, sometimes I see it happen a few times a week, other times I don’t see it happen for a month. Apollo also usually chases her off afterwards. As an occasional thing this can be a normal part of simply maintaining the dominant and submissive relationship, but if it’s very frequent keep a close eye out, I’m not really sure when the level become excessive to need to separate.

Has this been going on since you adopted them or is this relatively new? One of mine is a stress-humper. If something really upsets him, his reaction is to hump excessively.