I've had that happen on an old craftsman 42" deck. A couple things I found to watch for YMMV ... TSC Blue belts seemed to hold up better than the cheaper ones found @ Home Depot. Also, check the spindles closely, I've had the mounting ears crack & you couldn't tell until a load caused the spindle to tilt which threw the belt & shredded it. Check the idler pully for parallel as they age, they can get out of line with the other pulleys & throw belts. Finally, check the spring tension on your idler. My spring was worn/weak over time, so slippage at start & under load would heat up the belt & then it would get worse & worse, then throw or shred.
Thanks, I haven't pulled the deck out yet. When I do I'll check the spindles and pulleys closely.