This is unbelievable incompetence. And how could fireworks have that much energy released so quickly? They’re hardly explosives normally, just fast burning. Something about being confined in the truck’s box?
I’ve seen other reports calling them home-made explosives.
I don't know what started it, some small spark, or maybe a hot exhaust pipe. Being in a confined space, packed in with other fireworks, the flame would spread to the next item, in turn igniting more and more fireworks. Those, then, would burn, and as more ignited, the pressure would build, igniting more and more fireworks, until the pressure was high enough to explode the truck.
Low explosives, like black powder, need a confined space to explode. Throw a cartridge in a fire, it will just pop like a firecracker, but in the barrel of a gun, the bullet will be expelled with enough force to go half a mile.