There were certainly some unsual forces at play here. The angle of the tug, the flat tyre, the tilted tug cab (cabin) ...
Even if the driver kept his foot on the accelerator after the shear pins broke, why would the tug end up at that kind of angle?
The tow bar isn't touching the cab of the tug, so I assume the tyre of the aircraft did the damage to the cab: I assume there would be no twisting forces generated by the bar, because the (tug) end of the bar is usually pivoting.
I've seen the results of pushback/tow incidents, but fortunately none first hand. Some attributed to aggressive tug driving, forgotten steering pins, pilots putting on the park brake without being told, pilots not putting on the park brake (even though they said they had), tug driver heart attacks, tug drivers falling asleep, faulty tug steering ....