The belt has to run true on the alternator, the water pump, and the harmonic balancer pulley. Any one of them 'off' and the belt will fly off. The likely culprit is the alternator mount but I've seen grossly worn bearings on water pumps (yeah, they either didn't leak or leaked 'a little') that allowed the water pump pulley to not be right so check it as well as the harmonic balancer. If the balancer has lost its grip and is walking out (or in) it could possibly (but not probably) throw a belt.