Own Account Driver:
The sparks thing makes me wonder if the underlift failed and somehow the whole thing ended up sliding along like a sled.
In one of the pics/videos showing the front of the bus it looked to be standing normally on its wheels.
I’ll throw this one out for discussion:
bus becomes detached from towing frame for whatever reason. (grabbing brakes?)
Lets assume that both brake lines were connected to it.
Air lines ripped off. Yellow (service) line is now vented to atmosphere.
Wrecker driver depresses foot brake sending air to the brake chambers but instead it vents to atmosphere thru’ the broken line thus not applying the brakes?
Meanwhile reserve pressure would be dropping rapidly due to the venting red line.
I know a bit more about truck mechanics than some, but not as much as others. The air braking system is quite complex with many valves & sensors, I’ve never fully got the hang of it so will bow to the superior knowledge of a fitter who has been on an air brake training course.