Two stroke engines can easily start backwards and run that way, especially if they were overprimed as stated earlier. It happens to me a lot. I just slowly close the throttle with the idle trim on the transmitter and there is a point right before the engine dies where it will reverse all by itself and run forward. When that happens, I reopen the throttle and fly.

When a four stroke reverses, it's burning fuel that enters the muffler through the muffler pressure tap. All you can do with those engines is to stop it and restart after making sure that the fuel line to the carb is full of fuel. When these engines run backward, the carb becomes the exhaust and the pressure will blow the fuel out of the fuel line.