“What would make a truck catch on fire? Could this have been mechanical failure rather than the driver driving like a lunatic?”
I doubt the driver was driving like a “lunatic”. It is possible he was dead at that point. AS to your fire question the new emission systems on trucks can generate some high heat. That is also a possibility.
Something in his load for delivery? Run-a-way Battery perhaps? Pray for the families involved.
If the guy was a smoker he may have flicked a butt out the window and had it blow back in.