Your bun sounds like an athlete with good problem solving skills!
I have one bun who was a real jumper and climber. I ended up putting shelves on the wall in a stair pattern so he could sit in the window, which was what he wanted.
Most of my buns have developed skills for getting over or...
Both my buns will engage in this mounting behavior (I have one neutered male and one spayed female). In my experience, they do this when they are a bit afraid/threatened by an object.