Our first long trip with our new 32' bumper pull toy hauler. TV was an F150, which the salesman assured us was plenty for pulling the trailer. We made it from Cheyenne, WY, to Rock Springs, without too many issues, other than feeling some sway, which we thought was just because of the wind. The next day, we went up Hwy 191 (2 lane highway) north of Rock Springs. Windy day, as usual, but a tour bus went past us, immediately followed by a semi. The trailer started rocking and rolling and we pretty much thought we were going to lose it.
We pulled over, the hubs took a breather, and we kept going on to his dad's cabin in Pinedale. We ended up leaving without the trailer, his dad brought it back to us with one of his company trucks later on.
Basically, it was caused by a too small tow vehicle, and the fact that the dealership did not show us how to hook up our anti-sway bars properly. The way they showed us, basically made the bars useless. After that trip we got on youtube and taught ourselves lots of things, and the hubs got a bigger truck (first an F250, and now has an F350 dually). He's also got some more miles under his belt so he knows more about how to drive when sway does happen (because it's going to happen on a bumper pull toyhauler of that size). And, we go slow. I think one of the biggest causes of these accidents is speed, we've seen people fly past us going 70-75mph and I wonder how they haven't killed themselves yet.