Rabbit dominance behaviour
The mounting, circling, chasing you mention is quite natural to rabbits and is a way of asserting the dominant and submissive partner. It is not bullying in the way we imagine it.
How old are the rabbits? I assume they are from the same mother. Are they the same sex? If not, you need to be very careful otherwise you will end up with lots of little rabbits.
I assume they are not yet neutered/spayed. You should go to 'rabbit care' on this site and read the 'neutering/spaying' page. This will explain all about the behaviour and when you should get them neutered or spayed. If you do not then they will eventually become aggressive.