Shredded serpentine belt

[8cam]

On my way to meet some friends this afternoon and I had my very first breakdown. My own damn fault. I had noticed a squeal when I turned on the a/c for the last couple days, but didn't think much of it because it quieted down right away, figured I'd get to it this weekend. Well halfway to where I was going, and it didn't squeal, it screamed, even with the a/c off. I pulled into a gas station, popped the hood, and was greeted by fragments of belt everywhere and about 2 strands still on the pulleys.

Needless to say I didn't make it to the meetup. I Uber'd to the nearby Napa, got a new belt, and did a roadside replacement. All is well now, all pulleys are turning perfectly with no discernable wobble, and all belt-driven stuff is working perfectly.

I've always thought the tensioner vibrated more than it should, but it was rebuilt by Marc 20k miles ago and everything has been fine. But 20k on the belt and a catastrophic failure, luckily in a place where it was easy to pull over and take care of the problem. But what in the world could have caused it?