I'm sure we'll never know the answer for sure, but with the bucket way up like that it's got to have a much higher center of gravity than if it were down low. I'm just wondering if it were lower, whether it would have been more of a sideways slide, rather than a roll.
The other question I had was, if it did happen slowly (which, it may have just seemed to be happening in slow motion, as these things sometimes do), whether he could have saved it by a timely swing--either downhill, to catch it before it rolled, or uphill, to put more weight on the uphill side, again, keeping it from going over? Or would that have just made it worse?