It didn't fall off, it probably flew off. One of the pulleys it rides on, beit power steering, A/C, crank, alternator or the idler pulley may be going bad. The idler pulley which is made to keep tension on the belt sometimes gives out and the belt gets loose and when it does it flies off. High revs can do it also but I doubt that was the case. Look for wear on the belt, and then make sure all your pulleys are working smoothly and are aligned. If you haven't replaced the serp belt lately, now is a good time.