There is a force acting on the hitch besides the down force of the hitch weight. That force either pulls away from the vehicle, as when accelerating, or pushes toward the vehicle, as when decelerating. Depending on where that force acts, down force can be added or removed from the front wheels in either case. In your low hitch stump scenario, the front wheels cannot lift because the pulling force is acting below the center of rotation of the tractor tires, levering the front wheels more firmly onto the ground. The rear wheels will slip when enough torque is applied to them. Hope this helps.