Years ago Jack came across this load that fell off a truck north of Coldfoot. Click on the first one and scroll to the right. More info on the captions.
Jack is pretty sure this is an “explosion box”, a portable unit that’s probably used as a break room next to a rig most of the time, but it’s most crucial use is as an explosion shelter. If there’s a an explosion individuals can stay in it for protection. It’s actually upside down here and the damaged attached boxes are probably heaters or ventilation units.
The driver was looking to the left, drove off the road on the right, then turned the steering wheel abruptly to get back on the road, breaking the chains that held the box down and tossing it off the trailer.
Marks where the box first hit the ground.
This is most likely how the fallen load was tied down. This truck is carrying a similar unit.
There are four chains here. The fallen load was probably tied down correctly but the force was enough to break pieces of the trailer, probably not the chains themselves.
The Jack Jessee Blog 
Looks like someone had a bad day!
Hey, I bet Jack check his chains every day, but how often other drivers do it, for me looks like that chain broke too easily.
He says he doesn’t think the chains actually broke, that instead it was the aluminum trailer rails, but can’t be 100% sure. And now, he doesn’t have any chains, only a silver bullet filled with diesel. 🙂