The snap is caused by the wing stalling, as has already been mentioned. A lot of people have the mis-conception that a stall is always caused by too little airspeed. A wing can and will stall at ANY airspeed (assuming it doesn't fold first) if the angle of attack becomes too great. That appears to be what's happening with your plane. Several set up parameters can affect this problem, but for a simple solution, just don't pull a whole lot of up elevator in a hurry.