The most usual cause of engines starting in the opposite way is because of over-priming. This can happen from fuel syphoning from the tank if it's left to sit for a while or possibly the refueling pushes some fuel through the carby. Normally it's simply from priming too much though. It's not dangerous to the engine and pinching the fuel line to let it use up the excess fuel that's inside the crankcase will cure it. If the engine stops then don't prime it anymore, just flick it or give it a bump with the starter.