What causes trucks to burn?

I believe diesel will burn very easy, try it. It's not as explosive a gasoline or burn as fast or as hot.

 

If you get a substantial enough hit for that to happen, you're creating a lot of friction... friction creates heat, and flammable materials will catch fire if enough heat is applied to them. Brake shoes, air lines, polymers and plastics... all of those can be burned.
As for bursting into flames the way you describe, I've seen it before, but we were talking about KBR or Horizon fuel trucks which had taken RPG or IED hits directly. Even then, for them to just instantaneously explode wasn't the norm. I've never the sort of spontaneous combustion you're talking about from a collision.

First, diesel fuel has a HIGHER BTU rating than unleaded gasoline.
During the Russo-Finnish war, the "Molotov ####tail" came to be a thing. The Soviets didn't coin the term - the Finns did, as the Soviets claimed to the rest of their world that their firebombs were actually food drops. The Finns took a bit of grim humor to this, and named their improvised incendiary device the "Molotov ####tail" so that they could have "a drink to go with the food". The Molotov ####tail was a mixture of gasoline and diesel, rather than straight gasoline - the gasoline, for its flammability so they could ignite it, and the diesel fuel, because it burns hotter, once you can get it lit.
Second, have YOU tried it? If not, I'll make a bet with you... you stand in a puddle of unleaded gasoline, and I'll stand in a puddle of #2 ULSD. On the count of three, we both drop a match. We'll work out the details on what the survivor wins.

 

Diesel is not flammable it is combustible. But when pressurized such as an engine it is flammable or something like that.

 

Not pressurized... vaporized, after it passes through the injectors. If diesel fuel were so flammable simply for being under pressure, these aftertreatment systems would be roasting trucks left and right.
Diesel is still flammable, regardless... it just has a higher flash point than unleaded gasoline, which makes it more difficult to ignite. A match may not ignite it... put a cutting torch to a puddle of it, and you'll get it done.

 

In simple parlance, diesel is pumped to the engine via an electric pump and once it gets to the engine it is under very high pressure.
In a rear end collision, the fuel line is broken and this pressurized fuel in the system (before the injectors) easily sprays out onto the surfaces.
The engine is hot, there is lot's of friction and sparks in this kind of collision and the fuel catches fire.
Diesel catches fire and burns easily, especially after it's been vaporized, common when sprayed.
In many instances, the fuel line is ruptured and the pump continues to pump since it's electric and the batteries are not disturbed. (placement)
There are records where emergency responders have arrived only to find the fuel is being pumped STILL!
The pump is turned off via your ignition.
Diesel without tax is also called 'home heating oil' for oil burning furnaces.
You will notice tankers hauling diesel have flammable placards.

 

Flammable and combustible is splitting hairs over the 140 degree flash point at which the differentiation is made. Safety and compliance folks love to do that and argue that you have mis-labled something (OSHA and MSHA are notorious for this). Still catches fire fairly easily and supports combustion in a very hot way no matter how you split the hair. Cornstarch, grain dust, powdered sugar will cause a hell big boom. Google what happened at Dixie Crystal in Savannah, GA or some grain elevator explosions. That stuff has a helluva high flash point but it is such a fine particle and there are so many, it might as well be hair spray or other aerosol products. Diesel will still ignite very easily with a match or cigarette lighter. All you have to do is atomize it to a fine mist. Some really bad truck fires (burned to the frame) had nothing to with a crash. It was a vaporized stream of a petroleum product passing through a pressurized system with a pin hole leak that atomized it onto a hot surface like an exhaust manifold or turbo. Starts to cook on that hot surface til it reaches it's flashpoint. Then you got a fire - an extra hot one and a steady spray of atomized (diesel, oil, whatever) liquid that now (since we have open flame and it can act in the manner of anything that will burn like a dust) it like spraying an aerosol on a fire - only a really, really hot one (as Witching Hour pointed out about BTU), so now it's cooking MORE petroleum based products. And you're driving (flames driven back and down by the wind under the hood; smoke rolling back and dissipating fast cause your going 65) so when you finally get to see it, it's honking hot when you open the hood with a little ABC extinguisher in your hand and flood it with fresh oxygen.

Forget about it. Pop the pin and squirt it just to say you did but it ain't gonna touch it. Get away. Bad battery connections can do it too. There is some serious welding unit current pumping though the electrical in a direct short on truck batteries and they'll arc a fire which then burns through something that causes something to shift and all the gunk and goo on the lower engine starts cooking which then starts the afore mentioned process.

Just something to keep in mind when we just kick the tires on a pre-trip. Being guilty of that myself I have no room to slam anyone else that's done it. Oh, and no; I'm not standing in either wading pool, but I will say this: The gasoline guy's gonna go up with a big whoosh and the match will PROBABLY go out in the diesel. I ain't risking either though. I'll stand by and video it for YouTube. LOL.

EDIT: I'd listen to what Witching Hour has to say about things involving engines and what lines carry what to where and he can probably elaborate on my dumb driver stuff about how they can spray what on what a whole lot better than I can. But the general concept is atomized products with a higher flashpoint than gasoline burning like they were very low flashpoint products and please: no stupid experiments at home.

 

Most truck batteries are directly in front of ruptured fuel tanks. Also, a lot of trucks are essentially doing a rolling re-gen at various times and the mufflers and converters are extremely hot during these times. Much hotter then normal. Certainly hot enough to ignite fuel that is no longer confined to the tanks and lines.

 

Maybe newer trucks have an electric pump feeding the injectors,mine is still the old fashioned mechanical pump, but the law of the land (NHTSA rules) all vehicles that have an electric fuel pump that is in any collision the circuit must disconnect. The circuit is sensitive enough that a strong bump may disconnect the circuit disabling the vehicle unintentionally. That is why cars produced this way have an easily reached reset in the trunk or rear of the car.
But as been said, there is plenty of diesel fuel vapors around, ruptured tanks, battery boxes nearby which probably go to a grounded circuit at the chassis with a thick (3-0) battery cable providing a significant spark and heat. Lots of plastics on newer trucks light up pretty easily, and aluminum when it gets hot enough eventually catches fire.

 

Explosives!

I have yet to see a truck burn from being hit in the rear, except in the movies. There if you run over a curb your truck explodes along with the 2 trucks on either side of you.

As far as cars not exploding.... Look up the Ford Pinto!