This would occur if driven w/ the tension pully not secured. The position is the identical to the position when secured except pully drag can now overcome the spring return load allowing the inner belt surface to loose it's grip to pully surfaces. The belt begins to melt smooth spots across it's traction surface (Glaze)becoming increasingly thinner, weaker, brittle, and less functional eventually. I don't recommend pushing it till catastrophic failure. They seldom go quietly and don't mind takeing something else w/ it when it goes(yep.maybe even you). Worst I know about, 3 am, in the middle of the desert, 70 mph, 4 passengers, loaded to capacity (the truck), pulling a 21 ft. boat. As the belt exited the engine compartment, it became wraped around the left front axle, locking up the tire and all passengers were then treated to Being awoken from a dead sleep, while an unscheduled 540 degree panoramic veiw of the local scenery was already in progress. No one managed to get hurt and he had a new belt stashed behind the seat he had intended to install after returning from their trip, but had to be towed to a shop anyway because they couldn't get the belt dislodged & the L/F Brake line was now missing a small section. Anyway, back to yours, It could be the sum total result of multiple influences. Pully misalignment, excessive drag due to an internal binding within a component including bearing failure, Stripped set bolt on tensioner. If you don't know what is or isn't within acceptable tolerances for pully alignment, drag, shaft movement w/wo bearing slop, by sound,feel,and/or visual inspection. Find someone that does to change the belt and go over each components inspection with them so you'll then know what they're like when their OK and/or not. *Note: On Pwr/Steering pumps. While looking for a new unit to replace mine, i inspected pumps from 5 different makers at 3 different suppliers and found 3/8" inch or more in their shaft movement front to back, w/each suppliers assurance that this was normal, except for Motorcraft w/aprox. 1/16. Roll your old one up, seal it up in a ziplock and toss it in your trunk for good luck | 
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