How fast do you think he was going? It's hard to tell, but if the white line in the road is painted similarly to how they are in the US, he had to be doing 80-90 mph when things started going awry. I'm going to go out on a limb and call that the primary cause of the accident.
I agree the other items mentioned factor into it, but in the end, if your rig is only safe for 55 mph, then you should only be doing 55 mph. If that trailer is swaying you need to slow down. When I had the tow dolly, I limited myself to about 62 mph for this reason - anything above that and there was sway. Some days I had to go even slower.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab