kviper is correct, you sucked your belt in by coming on to the brakes hard and at speed. The noise does sound mechanical so you would easily assume it is possibly the chain case. If it happens again, pull your side panel off and take a look at the position of your belt. You will have full understanding of what is occuring, simply grab the belt and make sure it is sitting normally on your clutches and off you go. You more than likely burned a flat spot on your belt thats why it blew off on you later.
This has happened a few times on my apex when a situation occurs that i have to come hard on to the brakes usually just a couple of quick chirps on the throttle loosens it off but twice I have had to loosen the belt by hand. I figured it was ok as the tree ahead of me probably would have won the battle!