When the leaf springs compress and decompress.
I believe the torque tube and the rear axle moves front to back and back to front.
If you think about the fact that the front of the rear leaf springs are attached to the frame using one eye bolt.
The front of the leaf spring is just swiveling on that one eye bolt.
The back end of the rear leaf springs are attached to the frame using shackles.
When the springs compress and decompress the axle is moving slightly back and forth because of action of the shackle that uses two eye bolts.
It is swiveling on two points.
Photo of Rear leaf spring and Shackle 
