I can sit on certain inclines and not move and then other I do.
Could depend on the surface as well. The wheels could have been in a position where they were stable enough not to allow a roll.

 

There's no damage, so it's not worth worrying about. Why would you try to roll back if the other car was already on your ass? Let's say you were imagining things and the other car never touched you. Now, if you let your car roll back and accidently hit the other car, you're going to be worrying about a lot more than just some imaginary tap that left no marks on your back bumper.