Why does your betta do backflips?

Answer 1

This depends on what you mean by backflips. If it's just a way of swimming, but otherwise your betta swims upright and normal most of the time, then it's just being a fish.
If your betta is stuck upside-down, it could be related to water quality or a swim bladder issue.

Answer 2

It depends........... If the filter blows bubbles he could just be investigating. My betta always investigating my bubbler in my tank, in my opinion he enjoiys them :P