Delancy, The 99s had 6500lb towing weight.
That's a suggested weight used in commercials, not a tow rating, and best used to determine how big a boat can be safely hauled.
The trailer looks to maybe had another 1000lbs of parts on it.
I bet there's more there than 1K, but irrelevant because the weight is on the tongue, which has a maximum rating of 750lbs listed.
Add half of the 4k of the 40, because the weight of the engine is in front of the front trailer axle, and it's like a giant, unpredictable lever pushing down on the ass and of the tow rig.
So it should have been below the trailer towing weight of the 100 and probably the trailer too.
100 GVW 5k
100 GVWR 7.2k
Assume the trailer has an adjusted (down) 3,500lb capacity per axle, that's a 9,200lb payload capacity.
This load is far less than 9,200Lbs.... 9,200lbs is a lot more than the 6,500 tow capacity, but can guaran-damn-tee you'd have to be a seasoned hauler to safely travel at the limits of what's posted on the pillar.
Just because it says one thing, it isn't a blanket statement that covers all variables.
Loading says it all, not only weight distribution and securing to the trailer, but a trailer that's structurally sound.
The other factor is braking, and the trailer doesn't have.
It does not seem like it should have happened as the second tire should have balanced the load to a stop.
Especially a rear trailer tire, which assuming this had to be, because the other three in the pics aren't blown out.
When reading west of OKC, immediately assumed the section of roller coaster highway at Tinker, but this stretch is flat and featureless.
Doesn't matter and not wanting to add insult to injury, plus appears Tucker and Roma are well on their way to rebuilding, so nothing can be added beyond condolences to them.
Hopefully others seeing will give consideration to safety, and purpose of my diatribe.