What would cause a drive belt to fail like this?

[dieselreaper]

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This was changed around 30,000 miles ago at the same time as the cambelt and water pump. I heard this strange noise while driving yesterday that sounded like a loud flapping that got louder with revs (but silent on idle). Before I could pull over safely it had stopped and I thought maybe a plastic bag had got stuck under the car and carried on driving for a few miles. When I went out again later the battery light was on so I turned around and went home. The drive/auxiliary/serpentine (or whatever it's called) belt was shredded and off its pulleys.

Does anyone know what I should be looking out for that could have caused this before I get it replaced? The alternator pulley seems to spin fine but I didn't have time to try anything else (the rest is pretty hard to get to from above). Cheers.

(2012 Passat B7 TDI 130 saloon by the way).

[Crasher]

I would say the alternator one way clutch is seized, the way to test this is spin the alternator and grab the pulley to stop it, the internals of the alternator should carry on for a moment.

[dieselreaper]

Cheers for the tip. I've been reading that I can jam the alternator with a screwdriver, then try to turn the pulley both ways and it should turn anticlockwise but not clockwise. If it turns both ways (or won't turn either way) then it's bust. Sound about right?

[Crasher]

Yes, more counter hold than jam and use a nonconductive tool or disconnect the battery.