Wind.
I once spent an extra 2 days at a KOA near Rapid City, South Dakota due to high winds.
Finally, we had to get going.
Between the campground and the freeway, there was a 53 foot semi on its side.
We fueled up at Flying J, taking note of all the trucks and RVs sitting there waiting, wondering if we were doing the right thing by continuing. As it turned out, we were NOT doing the right or the smart thing. We made it, but it was a struggle. Within 5 miles of the Flying J, Westbound, we saw two more semis and a fifth wheel RV laying on their sides. The cross winds almost blew us into one of those, but I managed to keep the coach on the road.
Between Big Timber and Livingston on I-90 there are often high cross winds. It is not at all unusual for high-profile RVs and semis to get blown off the road.