[daepp]

If the Fedex truck was already on fire then my theory is wrong.

I am also assuming the tractor on the semi was diesel, so I wonder if maybe his brakes could have ignited?

You just don't very often have fires like that. I have commuted in So Cal for decades, and short of an engine or electrical fire - which are almost always very slow to progress - you just seldom hear of such horrific burnings. Horrible accidents yes - but if not killed on impact, most get out in time. This one seems to have ignited immediately.

FWIW - just speculation on my part. I have not training in any of this.