Had a similar experience with one of my "other" cars (which doesn't have a spring loaded belt tensioner).
I installed a new belt (Gates) with not enough tension, causing the longest span of the belt to vibrate like a banjo string, jump a groove & rip off one side of the belt. After a few choice admonitions; I replaced the belt again with a Dayco, adjusted for more tension & haven't had a problem since...
Maybe not enough tension?....my .02...
