I know the exact spot where the OP went over. It on the last curve before a long straight away. It's next to the Plaster city narrow gage (3') railway. In years gone by, it's just a slightly undulating sandy desert RR service/BLM road. Me thinks the OP was inordinately unlucky and simply got into the 'perfect storm' as mentioned above.
I use Happi jax and find them well engineered. The fronts are spring loaded at a 40 deg. angle or so and that's to keep the box on the bed (and no slide out the back) up front where most of the weight resides. The plate tied to the bed is vertical so lots of sheer. The rears are tied to the factory bumper, which, on a Dodge is pretty sturdy. i cannot vouch for Ford or GM. It is made to flex the bumper, with no spring utilizing the bumper instead. I've heard that some mfgr's bumpers have a one-way trip with flex: Up.
There is a lot less stress on the rear tie downs.
IMHO, THE most glaring mechanical misjudgement in this scenario is the lack of centering guides on the truck bed. In nasty situations like this, much of the sheer could have been transfered to bottom and side plates of the TC and to the centering guides making it difficult, nigh impossible to slide sideways at all and snap the tie downs. The wind would have to crush the bottom box of the tc or lift it straight up at least 6 inches to launch the thing as shown in the pix. There was obvious shock loading going on at the start of 3 seconds of all hell breaking loose. This is a basic tenant of my XTC spiel: the rougher the road, the looser you run the tie downs. This ONLY works if you have centering guides that act as a keyway. Transfer that sheer.
O.K. , so I still had a probably typical response and said, "Oh No!", over and over again as i read the post. It's easy to pontificate if you were not on site and just looked at a few very descriptive pix. The other thing is: a hardside is luckily (maybe not in this case) quiet and noise resistant to things outside, as are modern pickups. I can see why the OP had no idea what was happening.
I'm glad all were safe.
regards, as always, jefe