We're assuming this was not caused by another vehicle. But we don't know that.
One of the largest threats to a truck towing a trailer is what I call "Nirvana Syndrome". People see a trailer, and regardless of the speed it is travelling, people assume they are being denied access to a Driver's Nirvana that is directly ahead of the tow vehicle. They desperately seek to enter Nirvana, and will do anything to get there. The first idiot takes the first crack of space he can get to go around the truck and trailer, but his line of sight is blocked by the trailer. Once he gets around the trailer, he immediately realizes that Nirvana is not there. Just more traffic. By this time, Idiots #2, #3, and in some cases Idiot #4 have also attempted to enter Nirvana, also discovering just more traffic. Idiot #1 slams on his brakes. The rest follow. Truck and trailer either end up mowing down the new set of idiots, or he's naive enough to think he can turn out of their way. Result is the pretty much like the pic OP posted, or a jackknife.
I've also experienced this problem when driving either of my wagons.