Wheel hop followed by some backwards motion, and improper steering which lets the front end get out to the side. Once that rig get about 45 degrees on a slope that steep, the engine is trying to push the front end down hill, and there's no rear end behind it to stop it any more - it just gets pushed out of the way.
I agree with many here low-speed rollovers at nearly ground level in the never K5 is pretty uneventful, and I'd offer that if you were going to put in a roll bar, you also replace the seats and belts to reflect the expectation that you'll roll over. Everything is a system - can't just change one part without consequences elsewhere. Also, if you don't cut out weight (interior metal) in the cab to make space for the cage, you are also raising the center of gravity, so might be increasing your likelihood of rolling over. If you do decide to build a cage, pad the heck out of the parts of the cage where it is possible. And if you run one don't forget the PPE... I rarely see off-roaders using helmets except when they are paid too run. It doesn't cost much and could save you from hitting your head against the rocky ground during a roll-over my last
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