Sidewalks are built for pedestrian traffic, not to support the weight of an aerial lift. The wheels of this aerial lift exert hundreds of pounds per square inch of weight on the ground beneath it.
This operator chose to operate the aerial lift on a sidewalk which also contains service covers. Obviously, these service covers are rated less than the sidewalk. The operator drove over the sidewalk cover causing it to crumble and ultimately triggering the toppling of this aerial lift.
The condition of the occupants of this aerial lift is unknown; however, the most common cause of fatalities in aerial lift accidents is turnover.