Semi plunges off overpass

Could be he landed on the side of its gas tank and the sparks from the metal hitting the ground caused the fuel to ignite.

Diesel isn't ignited by sparks but by pressure and heat. Suffice to say falling off the side of a bridge going 50+ mph is probably enough to produce sufficient pressure to ignite it. Seeing as these vehicles carrying hundreds of gallons of fuel, a fireball isn't too big of a stretch. But I guess I'd still like to know what he was carrying.