This morning whilst reversing from my drive I noticed the alternator light come on (2002 A170) Then the steering went stiff. On checking the alternator drive belt had come of its pulleys. The belt looked perfect and as near as I can see so were the pulleys. After alot of swearing I managed to refit the belt and all seems well. But what happens here ,why would it decide to come from its pulleys. Whilst running you can see the tensioner pulley virbrating back and fourth very fast and I belive this is the noise I hear at idle. I intend to fit a new drive belt but I want to know what happend here,is this common?

All the best Ken UK