Load crashes off truck going downhill - GIF

[neilbourjaily]

It looks to me as if the drag tractor got off line in the first few seconds of the video. It appears the front end splays off toward the camera just as it is cresting the rise.

[KustomsbyKent]

This isn't too surprising at all. I lived in China for 5.5 years, and it was pretty common to see heavy equipment on a flatbed truck with no chains, and rolling good speed down the highway. If the flatbed was empty, they'd just throw whatever wood blocking on loose, and for the most part it'd stay on.

In this video, it also looks like there's a truck/trailer right at the bottom of the hill, which is probably why the lead truck slammed on his brakes to stop. Once those brakes locked, and the wheels started to skid, it was all over. Sliding friction is much lower than rolling friction, so once it started sliding, it was able to keep moving.
As Frank mentioned, lack of tie downs, too much weight on the trailer vs the trucks.

Another thing I noticed about the trucks and trailers in general in China was that their condition is usually quite poor. They run them very hard, extremely overloaded with poor tires. Most of the lights don't work, shields and guards are gone or baling wire to hold them on, poor welds on any repairs.... so its quite possible that the brakes on that trailer just don't work.

Preventative maintenance on trucks is just not part of their thinking. I can't count the number of times I saw a heavy truck on the side of the road with the driveshaft end on the ground because the u-joint in the driveshaft blew. Rather than fixing it before it blew apart, they'd wait until it broke completely, then a small white repair van would show up, and fix it on the side of a busy highway.
I've also seen an engine being overhauled on the side of the highway, and transmissions as well.
We'd always be driving on vendor visits to manufacturing places, so I'd see a lot of highway time. Unfortunately I wouldn't always be able to get my camera ready in time.