I am amazed and impressed that the driver could walk away from that. His cab must be very well insulated to protect him.
If he was in the cab when the whole thing went kaboom, I wouldn't think he would have been able to walk away. Even if he was protected while in the cab, he'd be unprotected as soon as he left it.
The only scenario that I can figure that would make sense (other than malfeasance) would be if the truck had an engine fire and the driver attempted unsuccessfully to put it out, suffering some minor burns in the process. When it became clear that wasn't going to work, he left to summon help and then the whole thing blew up; if he saw a taxi at the gas station, he may have hailed it to the hospital as a faster alternative to calling and waiting for an ambulance.
Still leaves open the question of why the truck would be under the bridge, though.