Firstly, let me say that I am not a tug pilot and so would not attempt to hazard a guess as to why this happened. However, from the video it appears that the aircraft has an increasing nose up pitch attitude as the pilot applies full back stick and then the aircraft rolls left, possibly due to a stall leading to a wing drop? It is interesting that the pilot closes the throttle as the aircraft reaches the maximum nose up pitch attitude then moves it fully forward again. Very sad.