[Richard]

It's not too hard to picture why it does that. The drive wheel is locked rotationally to the drive axle. Just for demo, draw an imaginary line up and down on the wheel. Now picture rotating the swing arm that connects the chassis to the axle as if the coach were moving up and down. The wheel, being locked to the axle, wants to rotate in alignment with the swing arm, but it can't cause the coach weight is holding the tire on the ground.

As Bill has pointed out with two data points on his coach, this situation can apparently bind the caliper pins, keeping the pads firm against the disc if the bind is not released.

I am thinking this is much more of a potential problem with coaches with rear disc brakes than drum brakes.