I would be interested in seeing how that trailer was packed. Most likely a large Tool Box loaded to the hilt plus a front engine car of some sort. The back end between the tail of the car and the trailer door is probably just a bunch of light stuff that is easy to pull out.
The interesting thing is that he has his tongue extended (I have a similar setup though not as extreme and I'm not hauling nearly as much weight) so that would reduce some of the loading on the hitch and put more weight back onto the trailer tires. However, it also creates a very large moment arm so while the static downward force on the hitch is less than if the tongue was shorter, the twisting moment on the tongue could end up being more if that trailer is poorly packed with all of the weight up front.
My guess though is that the combination of a heavy tool box plus a, front engine car put a lot of the weight on the tongue (as opposed to over the wheels). Hitting a few bumps that are in the same resonance frequency of the truck frame working as an un-dampened spring was to exert enough force to cause the frame to crack.
I've encountered one section of road where I could see this happening like this. The dips were almost imperceptible, certainly not really visible to the eye on first casual glance. I hit the section doing about 45mph and it caused my frame to oscillate enough to have the top of the cab hit the bottom of the camper overhead when I had 3" to 4" of clearance. It only took about three or four bumps to cause this and I could see how an even heavier setup will cause the frame to fail.
The real story is, how or why is the guy still driving? We don't have the timeline between the pics / video. I can't imagine someone with that nice of a setup not caring that his frame was twisted in half. One could only imagine the swear words coming out of him once it happened the second time and he realized it.
