First, once a belt has come off, it is much more prone to repeating the performance even if the belt looks fine visually. I think something breaks inside the belt which makes it wobble (BTDT). Second, misalignment can cause belt problems, but IME it takes a fair amount if the belt is new and the tension is correct. If you can lay a straight edge across both pulleys and there are no gaps, it should be good enough. Finally it doesn't take much vibration at the right frequency to get a belt to whip around and come off. I've read of cases where the belt problem was a symptom of a crankshaft counterweight problem (stuck). Try to watch the belt while the engine is running at various RPM to see if it starts to move up and down.