Timing belt "shredded"

I just replaced this darned thing about 6-8 weeks ago. Everything went well. Now, after starting the car yesterday, noticed a rubbing and "whirring" noise. Upon having a look I discovered shredded off peices of the timing belt under the car. Belt wasn't completely broken (thanks for small miracles), but I am wondering what would cause this failure mode ? I replaced everything... belt, water pump, idler, and tensioner. Only the innermost 1/4" or 3/8" of the belt looks like it is being "cut" . 4000 or 5000 trouble free miles since the replacement, now a sudden failure? I don't get it.