why does serpentine belt tensioner tighten belt to max, causing belt to shear

And the car is?, shredded belt, torn, ripped, does the tensioner move freely, all the pulleys lined up? Any other pulley driven accessories checked for binding. Belt the right length and width? Very short on info here!

The manufacturer designs the serpentine belt idler pulley to provide the proper tension to the belt . It will lose some of it's tension as it ages . Perhaps that is what happened - it allowed the belt to slip until it shredded ? Have you checked all the involved pulleys to ensure they are sound , properly aligned , and not damaged in some way ? How old was the belt that shredded ? Perhaps it was at the end of it's useful life and needed replacing ? MaxWedge is right - more info would be helpful !