I'm pretty sure the fire was caused by trailer brakes, either he forgot to release them at last stop, or leak or other reason they engaged, all the tires were on fire when I arrived on the scene. I did consider possibility of the brakes engaging and forcing him to stop, but do not see any skid marks, so its hard to say. Nothing but open highway north of here, I think it was one of those situations that with enough airflow the flames were kept minimal, as he slowed down coming into town the flames grew until he, or maybe the truck behind him could see the flames. Don't think it was intentional, just poor judgement.
Friend burned a truck and trailer to the ground once, he forgot to release trailer brakes, by the time he noticed he was on fire the trailer was burning well, he stopped and pulled pin to drop trailer, but I'm guessing lost too much air and then the truck would not move, it was a total loss.
I was indeed looking at the pictures to see if there were skid marks. Given that the weather was very cold, it could be that the flames developed only after he stopped.
It looks like you arrived on the scene rather late. It is all possible that just one brake was overheating and the other tires caught on fire later on. Especially if only partially engaged (e.g. leaking diaphragm of a spring chamber), it's somehow difficult to notice it on a heavy load.
It's very hard, if not impossible, to see smoke in your rear view mirrors during the night.
If the trailer had an air leak when it came to a stop and all the emergency brakes became engaged, it is practically impossible to move it anymore.
One thing that surprises me is that the driver took the time of lowering the landing gear. It doesn't make much sense (who cares if a doomed trailer get further damaged by dropping when the tractor moves away from under it), or it fit the profile of the stupid driver. But, again, it's hard to judge, not being in that situation.
At least for me it's impossible to judge if it was stupidity or the perfect storm. I would bet that he has a long list of questions to answer to DOT: an audible air leak, malfunction of emergency brake valves, inability of the compressor to fill the air tanks to maximum pressure within a given time, etc. are all out of service conditions. Each commercial driver is required to perform a pre-trip safety inspection before starting driving with the vehicle (and every morning, in multi-day trips), and a post-trip inspection when he arrives (or at the end of each day). Those inspections and possible defects must be logged (nowadays electronically).
Now, let's clarify a little bit the basics of how the air breaks are supposed to work on a tractor trailer.
The two air brake circuits are kept between 100 and 125 (in some trucks 150) psi: when the pressure drops below 100-110 psi, the compressor kicks in.
If the air pressure in either of the two redundant systems drops below 60 psi, a buzzer sounds continuously and a red light shines on the dashboard. If the pressure keeps dropping, any time between 40 and 25 psi the two emergency brake valves (first the red trailer valve, then the yellow tractor valve) pop out, closing the air supply from the air tanks to the spring brakes and bleeding any extra air downstream, resulting in the spring brakes on one or more drive axles on the tractor (steering wheels do not have spring brakes) and the brakes on one or more of the trailer axles to lock up.
The trailer valve has also another function: if there is a massive air leak on the trailer (e.g. emergency brakes glad hand coming loose, trailer emergency brake line severed, rusted-out spring chamber exploding, etc.), the valve pops out immediately, preventing the tractor's air system to lose too much air.
The emergency brakes of a truck are also the parking brakes: since they are activated by springs, they do not need any air supply to work making them perfect for long term engagement.
One inconvenience of truck's drum brakes is that, with a little rusting, cold weather, etc. sometimes they don't release after being engaged for long enough.
Regarding explosives, their transport is highly regulated (on the top of the normal hazmat rules, the driver must follow a filed route, stop only at specific locations, never leave the vehicle, call specific numbers in case of delays or breakdowns, etc.).
For DOT explosives are classified according to their reactivity (1.1 being the most dangerous and 1.6 the least reactive). They are also divided in-I believe-13 compatibility groups, which dictates what explosives could be shipped together on the same truck. If I remember correctly, any "primary explosive" (compatibility group A) can be shipped only with other primary explosives and nothing else. Blasting caps belong to compatibility group B (they can be of different classes, mostly 1.1 or 1.4, depending upon their reactivity). So, I confirm that they cannot be shipped together.