asshat

Well last nite i was driving my car and it sounded like it was starting to spark knock. But as i got home and looked the right side of the timing belt was fraided and preety much missing. So now theres like only 3\4 of the timing belt left. Does any one know what may have caused this. And yea im replaceing it and i do know what happens if it breaks so i was lucky it didnt. This happend under normal driving