Driver error.
You can see the smoke pouring out of the truck.
When facing a long downhill, the driver has to downshift prior to hitting the downhill. In this case he probably wanted 6th gear. You let the transmission keep you slow on the hill.
If you leave it in tenth gear, the truck will go too fast, and you have to try to slow down with the brakes. The brakes will catch fire, and the brakes will fail, and then, you have a runaway truck.
