I can tell you what happened and it really isn't your fault...or a defect....
Somethng got in there just long enough to make a cut almost 90% across the belt, then it was slung out quickly. Then when you hit the power agian it just tore the belt down the length of the teeth until it finally gave way. I have a standard belt that got that tell-tale cut.... I also had an HPI belt do the same thing a few years ago......