2001 Chrysler Town And Country Serpentine Belt Came Off, Why?

I have a '99 Town and Country. I had a problem with the belt tensioner working correctly, such that my belt would squeal sometimes and one time did come off completely. As I recall, I had the belt tensioner replaced a couple of times til the problem as fixed (~50K miles ago).

Mine came off twice. The car is a 2000. A friend said to check the belt to see if it has stretched. Otherwise, I cannot figure it out.