^^ You didn't set the belt tension up properly, allowing the belt to slip 1 tooth when the belt sags when the engine isn't on compression.
Magna's will not bend at 1 tooth, I've had 3 apprentices fuck up Magna 3.5L belts, one even slipped 9 teeth on the rear bank! Replaced the belt and it ran perfectly. 2 teeth wont bend valves, it will make it run like shit though.
It really comes down to luck, exactly where the piston/valve is when it happens and if it is auto or manual.