Question:

I took this photo at the bottom of a fairly modest hill. It was more obvious as we went by that the fire started at one of the trailer axels.
1.
Poorly adjusted brakes?
2.
Didn't understand the proper speed/weight ratio for using the jake compression brake?
3.
Rode the serivce brakes pedal too lightly, which only applied the trailer brakes?
4.
Going too fast?
Number 4 for sure, but probably some a combination of all above....

Answer:
Now that's a HOT LOAD!...and it still didn't get there in time!!
Just hope the driver came out ok!

Answer:
The good old red and white conspicuity tape sure holds up to heat well, don't it?

Answer:

You can't tell in the photo, but no tractor. The front hood of the refer didn't look like it burned at all, just everything behind it. Maybe the driver stopped, yanked the fifth wheel handle, and drove out before the tractor caught fire....?
(that's what to do by the way, if your trailer catches fire)
Nice little 48ft split-axle refer. Look at those shiny hubcaps......

Answer:
Most likely what happened was a wheel bearing had a melt down. Most new trailers have grease in the hub and when a wheel bearing comes apart it causes the grease to catch fire and that catchs every thing else on fire.
The driver does not always have to do somthing wrong to have somthing bad happen . What does the jake brake have to do with the trailer catching on fire. How do you know the driver was going to fast were you behind him? riding the brake with a little bit of pressure does not only operate the trailer brakes and is a good way to keep your speed under control on a hill. How do you know if the brakes were out of adjustment?
The only thing in your post i would agree on is if your trailer is on fire you get it off the road pull the pin and let it hit the ground no matter how big the fire is

Answer:
I don't know what caused the fire. It may have been a bearing.
But it was at the bottom of a hill -- not proof that braking had something to do with it, but a clue. He was obviously close to gross -- produce. There were no tire skid tracks, so it wasn't a locked bearing. But you're right, it could have come apart and caught fire without locking-up.

Many drivers unfamiliar with compression engine brakes won't shift down enough to make it useful. It doesn't do much unless you've got your rpms up. He may have thought flipping the switch and keeping it in high gear was enough while also riding the brakes. The preferred method is to shift down to a gear where the full 6-cylinder jake setting will hold speed without using the service brakes at all.

A little pressure only opens all ten brakes if you apply heavier pressure first, then back-off to a light pressure. Modern trucks have "proportional braking" technology which opens breaks at slightly different pressures. Applying very light pressure will open the trailer brakes first. The truck brakes open with a little more pressure. The idea, as I read, is to put tension on the kingpin during light braking in curves on slippery surfaces, rain etc.. As the trailer starts to pull back against the tractor in a turn, the trailer's drag against the truck helps maintain the kingpin angle and reduced the chances of a your drives sliding into a tractor jacknife.
I don't know which is worse, drivers who brake in turns and risk pushing the drives into jacknife, or a braking system that lets you fry the trailer if you apply the pedal too lightly for an extended period of time, but reduces jacknives. But all new trucks are now designed with "proportional braking", and it makes the transition from no braking to very light braking dangerous if all you do is just apply very light pressure without a harder initial application to open up all the valves first. Those of us who do hills a lot learn quickly when our slack adjusters are set perfectly, but the trailer still smokes with light pressure on a long hill. If you run mountains alot, you end-up using "stab" most of the time. Light, continous braking saves your tractor brakes so you've still got some brake if you need it, but may burn-up your tandems in the meantime...or may set the trailer on fire if one trailer brake is adjusted too tight and doing all the work.....and you're just plain going too fast.

Answer:
I tend to dissagree with the brake out of adjustment theory.. 1 tight brake is not going to cause your trailer to burst into flames, It will give you plenty of smoke before it gives you problems.
This looks like an O/O rig, chances are he knows how to use the service brakes, as well as the "Jake". He or she (gotta be politicaly correct) would not use the foot brake unless absolutely nessessary (I'm an O/O and I try and save brakes too..)
I tend to agree with the wheel bearing running out of grease / oil.
It would give off little smoke if it was out of fluid and with the chrome "high hats" over the hubs they were probably not monitored very closely. (Out of sight out of mind comes to mind..LOL)
Answer:
Good points. You may be right. I was thinking about why an O/O wouldn't notice till this late, but he may have had someone else driving it, or just been dead not paying attention. The thing is -- the hub caps are still shiny on the left side. Wouldn't a hub fire leave some trace?

Answer:
Looks like wheel bearing problems to me also. Most of us have seen lots of trailers that have burnt in the rear and some times consuming the whole trailer. Rarely does the tractor get affected if disconnected quickly.
Here's what flat ground can do to you when your out to late with the church group and only slept a couple hours. Falling asleep at the wheel is a bad thing. This driver barely got scratched. Possibly his Church Group has connections.

Answer:
From looking at the pic.
It looks like the fire started on the other side. (The trailer siding was exposed to higher heat over a longer period of time. Note the siding is gone on the passenger side)
So I am sticking to the bad wheel bearing / seal... Ya gotta take those high hats OFF once in a while and check that fluid level..
Answer:
Gonzo, took another look and I'm starting to agree. Probably the right rear. Wish we could se that side. I guess the last thing I don't understand is why the driver didn't notice the drag coming down the hill. For the friction to be great enough to generage that much heat, wouldn't the driver notice some drag? If it was you -- and even if you weren't familiar with that hill -- wouldn't you still notice that something was "braking" your truck, even slightly? The heat was so great it melted all that aluminum and fried boxes of moisture-laden produce. I'm not trying to find a way to blame the driver (I guess it sounds like I am but trying to fine a lesson in all this) but the bearing scenario would still have to produce a whole lot of heat, which means a lot of friction, which means noticeable drag.... ....or am I wrong about that on a bearing that goes that bad. Never had one -- fortunately I guess, since I don't always check trailer hub lubrication on the old hubs (that are driver serviceable) when I've got a trailer for a few days....
Check your hub lubrication. Is that it?