Since there are no tracks behind him, it looked like ha may have been backing down the road and veered too close to the edge. Stopped and lowered the chute then tried to dump part of the load but the accelerated drum rotation caused the left side to sink further until it went over.
These snippet videos don't ever tell us the whole story.
For all we know the operator could have been a disgruntled employee backed close to the edge dropped the chute spun up the drum and exited the truck then just let it sit there until gravity and nature took its course