I have see a motor reverse in flight that was to rich hitting the throttle just as it was quiting and poping the engine in reverse
We use to pop start a reversed running Hyper 50.....choked the carb with trim till the motor just about quits then pop the throttle and it would pop and run in the other direction...got quite good at it actually.
It was a head shim that the issue causeing pre-detonation because the head is lower to the piston letting the nitro burn sooner in the combustion cycle before the piston reaches top dead center. Driveing the piston down in the wrong direction making the motor to run backwards.The shim raises the head allowing the nitro to burn later in the combustion cycle after the piston has reached top dead center.
When they run backwards they make a compleatly different sound. No mistaking its running backwards.
If it didnt sound funny when you start it...then its probaly. Not running backwards.
Not sure that a failing OWB would cause the problem you describe.....maybe a failing clutch would be a suspect.
Fly Hard......Team Viagra