Auto trannys rely on the engine turning the torque converter to pump the
fluid through all the complex hydraulic valving and passages as well as
circulate the fluid for lubrication.

The towing certainly could have caused the damage, 20 miles is a long
way without proper lubrication in the tranny, regardless of speed.

The tow truck drive certainly knew he was damaging your transmission,
but also knew you would not be able to prove it.

Next time wait for a tow truck driver with some ethics, and the proper
equipment too.

Stuart