I would guess faulty brakes. It looked like to me that the driver put the truck into the guardrail and trees trying to slow it down. What would you do as the driver when confronted with no brakes and a road full of unaware parade participants?
Don't forget it's possible he a had fuel on board, however I did notice he left it pretty late with the horn I would have holding that fucker down.
Poor people no chance.
A lot of weight, downhill, airbrakes. Air brakes are not endless, they are recharged by the brake system, whatever the problem the driver blew the horn and road the guard rail to slow down. Very sad