I've built similar dynos for Superflow at the shop I weld at. When you look at the front view of the car in the video when the roll/drum comes out, you can see the mounting holes for the bearing mount and for the shaft to protrude from. Either someone didn't make sure the bearing mount bolts were tight after...who knows how many runs on that thing??....and it ended up ripping the loose bolts right through the holes.
Or, the rolls/drums weren't balanced properly....and at the frickin Autobahn cruising speed he was at, brought out the imbalance and it let loose.
I've balanced rolls for Superflow many times and it can be a PIA! It's done by computer but you have to weld the weights where the computer shows the imbalance is....sounds easy but it really sucks!! When the computer spins the roll, it only spins about 250rpm.
Lucky no one was killed.