Most likely they were driving too fast, or went to "sleep" at the wheel.
I tow a trailer, but I don't let myself get over 60mph, even though my 5.7L Tundra will easily yank it over 70. I just don't do it. 60 is plenty fast enough with an RV on the back.
The accident details are still under investigation, so I can't place any blame. However, the most common cause of truck/RV trailer accidents is driving too fast and getting a good dose of swaying, then getting yanked off the road by your own trailer.
I've experienced trailer sway once, and it was not good. You don't want it. We were loaded wrong and driving too fast.
