Same exact thing happened to my school bus in 9th grade.

going around a blind corner too fast (35) on wet leaf covered pavement, another bus and line of cars were stopped at the bottom of the hill. she braked hard, swung the rear end across the street, hit the curb, changing the forward direction to straight up a steep embankment. when the bus got too tilted it rolled on it's side. you think it's jarring to tip in an Xterra. Try doing it from one side of a school bus to the other sans seatbelt. ouch.