There’s a big difference between an unencumbered dump truck trying to slow down and one carrying a full load, as weight can definitely complicate things, especially if it needs to decelerate rapidly.
We don’t know if there was a problem with the breaks or a blatant driver error, such as lack of focus or perhaps sleep; your guess is as good as ours.
What we do know is that the accident occurred recently in Tomsk, Russia, and that 13 people were injured, though, thankfully, there weren’t any fatalities. We also know that the truck was carrying a load of rubble, which can easily add several tonnes to an already-heavy vehicle.
Another factor worth considering is the wet road, which certainly made slowing down even harder, not that an attempt to do so was in any way obvious, at least as far as we can tell.
Even after watching the video several times, it’s unclear whether or not the driver even managed to lower his speed. In any case, the truck started smashing into other vehicles, and would have probably kept going if it hadn’t flipped over, but other than that, it just looked to be completely out of control. This is, indeed, scary stuff, and it’s a miracle that there were no fatalities.