ts,, yes folks do not kno - understand the energy (mass x velocity) they carry and the potential (speed) that is on the very edge of control, ie out of control, and the impending disastrous results when soft bodies impact hard imoveable objects.
most likely that driver was barreling along (blissfully) thinking he was gonna outrun your truck to the squeaze point. kinda like folks who race trains to the RR xing? Upon loosing that bet perhaps he didnt realize that trailer was attached to your rigg and thot he could force squeaze between two moving masses.
The amount of energy his vehicle embodied was transferred into your rigg at a single control point(left rear wheel) and not the whole 4 wheel platform. you experienced loss of control as all that energy disengaged that left rear wheel(2x) and then loaded and unloaded the opposite corner. Of course the 5k+ trailer has its own set of energy and linkage to play on this scenario, and as you say was probably a stabilizing force.. Yes, very much like an old Sam Peckinpah movie playing out.
Our vehicles are so comfortable and automated, filled with all kinds of distractions, I call them Info'tainment centers. we have completely loss what little sense we mite have of what it is to truly PILOT a 3+ton cruise missle at high speed thru a maze of disorganized traffic patterns and other 3ton cruise missles.
And still yet everyday, there we are on our spiffy motorcycles, out in the fray, thinking mere leather and a helmet are somehow gonna save our ass from dimwits on their cellphones.. its really more like russian roulette with only one chamber empty in that revolver..
ps. good yor still here to write this report. pix would be a great enhancement. got any?