thats from getting it too hot. too much load. are you still running the stock secondary? did you hear the engine bog down any while you were at full throttle?
when a belt gets too hot, the glue that holds it together melts. when the glue melts, the belt pulls itself apart. too much load on the belt gets them hot like that. which usually is a result of the clutch being in too high of a gear ratio for the load and situation you were under. typically a stock secondary, wide open throttle, and hitting soft spots like silt or wet sand that bogs the motor down while at full speed. the secondary needs to backshift down a gear but the stock secondary just wont backshift while at full throttle.
adding an STM secondary to the mix would help solve that problem. or if you do already have an STM secondary, getting it dialed in a little better. the belts dont usually get all that hot when the clutches are operating properly. they will however get EXTREMELY hot when the clutches are under too much load from being in the wrong gear ratio,
if you would like to bring it to me some time this week i will be GLAD to give it a go and see if i can tune it a little better. i will be at glamis thursday-sunday tuning on clutches i can dial it in for you.