Tugs are governed at maybe 15mph. Standard regular conditions would take a hell of an effort to get one to flip. They are center weighted pretty good.
I have seen ulds and bag carts jack knive, and it never felt like the tug had a chance to flip over.
Gross negligence sounds most feasible.
Weather has some effect it appears. Now was the tug in solid condition? I have seen wheels fall off of tugs, Lugnuts sheered off. Both back and front wheels. Neither incidences had any casualties. We run these in snow too. You may see an occasional horse play slide in the snow with no cargo being pulled.
Pretty much, I don't see how this is easily replicated for normal law abiding drivers. Freak horse play more likely.