The mounting is a sign of dominance
- dogs do it, rabbits do it, etc. A male will mount another male to dominate it, and a female will mount a male or female to dominate it. Usually the dominated creature will accept it being dominated - but not always. Frequently it starts passive, then if the dominating creature continues the practice, the dominated one will begin to fight and you can have fatalities... Best to squirt some water on the dominating one when you catch it doing it to show it who actually IS
the dominating one around here! YOU
must be 'pack leader'