Are we talking the poping from the front or rear ?
U joints won't stop the MH from rolling, a stuck brake can.
With helper, get the air pressure up, and have someone push in the parting brake knob, while watching the rear slack adjusters. A normal one will move towards the chamber. A stuck one will hang out where it was before you pushed the brake button in.
As the shoes wear, the S turns more and more. At the last of the S, it can stick, as in the image.
The S cam shafts also require grease. The fitting is often overlooked and buried in dirt. If the dry out, that can also cause sticking.