I agree with the above but to add to that, some mowers are just funny that way. My Bob-Cat likes to throw the belt if I engage the blades with the engine rpms too low.
One thing that can make a difference is your cutting height and the resulting angle from the engine to the deck. Mid cutting heights usually have the least angle between the 2 and will result in less chance of throwing the belt.
Yeah, ideally it's best to resolve an issue but sometimes working around it is the practical route.
Edit:. Make sure that the spring-loaded idler arm that tensions the belt is lubricated and moving freely.