One would also likely need to calculate in the low pressure area generated on the leeward side of the camper, compounding the force of the wind.
In one of pictures there is dust blowing across the road/field. I lived in area's of Saskatchewan that is like that, one minute 40/50 mph wind gusts the next few minutes relatively calm. It's the nature of wind gusts in some locals.
As a armchair quarterback my guess is a combination of events to create a perfect storm. Wrong tie down system, typical poor truck camper tie down mount design/engineering, perhaps poor workmanship to boot. A big gust of wind and a little over correction of steering to keep the truck going straight (like skipro3 pointed out) and throw in a rut or two of soft powdery dirt. Walla the formulation for a 1 in a 1000000 flying camper. Or not?