Late 2018 year a buddy of mine was headed up to a car thing towing his 65 Healey in an enclosed trailer with his half ton. He'd been driving all day and let his co-driver take the wheel for a while so he could nap. Along the way he woke up to the co driver yelling with the truck was in full trailer oscillation and then him and the truck tumbling for a long distance with the co-driver disappearing along the way. Apparently his co-driver decided it was ok to go 80mph down a hill with the trailer, was not wearing a seat belt, got thrown out of the truck and killed. My buddy broke his clavical, arm, leg and a few ribs. The trailer oddly enough disconnected when the roll started and the auto brakes brought it and the Healey unscratched to a stop.
Was moving back to WA from TX, was going on probably my 15th hour of driving, about 2 hours left until I got home, pulling a 6x12’ U-haul trailer fully loaded. All of a sudden I started noticing a weird feeling like the truck was shimmying back and forth, I looked in the mirror and the trailer was going back and forth about a half a lane let off the gas, luckily it calmed back down, pulled over for a few mins and smoked a cig.
I saw a really similar tank slapper on the 5 a few years ago, the person passed me doing 85 on a 3-4 percent grade then tapped the brakes. The vehicle and trailer ended up sliding on their side for a couple hundred yards.
Had something similar (the trailer sway) coming down Snoqualmie Pass with my little pop-up trailer. To be honest, I was going waaay too fast for the conditions so that was the problem. Slowed down to a reasonable speed and all was good...