Ok, I just took the entire rear pulley/clutch assembly apart, and did not find anything wrong. That thing is a PITA to work on and I had to make a couple of new tools. I didn't find anything wrong or worn, though there was not much grease. I will put some more grease when I reassemble it. The clutch looks like it has plenty of life left in it. Most of the mileage was on the highway, not stop and go. It still has the original rear brakes, and the front pads were only recently replaced. So still no clues about what happened to the belt. I would say WOT throttle riding carrying a lot of weight might be the cause, but that would not explain why the other belts did not break. Also there is a ring of rubber around the outer edge of the front pulley, where the belt would have been most of the time, but nothing on the rear pulley. That also puzzles me. Up until now, belt drives have been totally reliable on scooters. Counting the Zuma 50, Metropolitan, Zuma 125, and Vino 125, I have nearly 50,000 miles on CVT scooters with no failures at all, so this is an oddity. However, I have seen a lot of broken serpentine belts and timing belts on cars. You would think they would last way better, because they do not move up and down in the pulleys. Scooter belts look a lot like timing belts except for the edges, which are tapered to match the pulley. Another thing is that this belt was torn to pieces all the way around, yet the scooter ran perfectly right up to the second it failed. It must have somehow got twisted up in the pulleys.