[ChevyCruizer]

Every action has an equal and opposite reaction! If i push on a wall it pushes back with the same amount of force that i am applying. Either the wall will move (unlikley) or i will move away from the wall equal to the amount of force that i am applying. Simple physics, same with the bag. It doesn't care how it is mounted. It applies pressure (force) equally. Since the trailing arm or axle is in contact with an immovable object (the ground) the other end of the bag has to move. Therefore as pressure increases the frame rises. Unless you overload the bag with too much weight or psi, then it ruptures.