The same exact reason that accelerating quickly causes far more tire wear than accelerating slowly, only in reverse. By applying more torque to brake/accelerate

The same exact reason that accelerating quickly causes far more tire wear than accelerating slowly, only in reverse. By applying more torque to brake/accelerate, you cause more lateral force on the tires (think, smearing the rubber itself) which is what causes tire wear. With more force the rubber is stretched further, which fatigues it and breaks off pieces of it over time.