'89 100: alternator belt... gone!!

[Andy Goldberg]

1989 Audi 100 sedan (oh, how I love thee :)

My alternator belt snapped off for the second time sice August yesterday.
The first time, back in August, was 2 days after I had a new A/C compressor
installed.  No problems since then, until yesterday, when the new alternator
belt snapped off.  Both times it occurred on a highway at around 60mph.
This time, it completely snapped, even though I pulled over right away the
belt was gone, no traces of it dragging along.

My mechanic was unable to find a particular reason, this time or last time,
although he's not an Audi or German car expert.

Any ideas, what should I be looking at as a possible cause?  Sounds too
strange to be a coincedence.

Thanks!!

-Andy
andynyc at spacelab.net