You have something Driving the belt to the back. that will happen of an Idler pulley is sitting on a bent stud, or the Adjuster has Krap behind it ( a Washer ? ) and is not sitting square with the block. these things will drive the belt to a direction and it will wear. If the Lower Washer on the Crank Pulley is installed backwards, it will shread the belt, but not force it to the back or front. Cj