Blossom my girl bunny humps her husbun Pippin when they play out on the lawn, I think Blossom is just excited that she has the freedom to run around.
I have 5 male/female pairs. All are neutered/spayed and only hump occassionally and mostly it's about showing whose boss. I wouldn't worry about it. It's very natural for bunnies to do this.
Rabswood