I think it is sparking before the piston gets to the top, and tries to force it backwards.
All engines do that. Setting the timing is what controls how far before the top it fires, measured in degrees before top dead center in the rotation of the drive shaft. It's possible that something slipped and it's firing even earlier.
Does yours have a compression release for use when using the pull rope? I had a Kaw that had one. Just flip a lever and it released the compression so you could get it moving without jerking your arm off. It was like the kick start bikes.
