A big gust. I lost a 5er in 2009, Elk Mtn Pass on I80 west of Laramie, WY. Lucky us. The trailer was torn right out of the back of the truck stretching open the 18,000# rated 5wheel plate in the process. It then barrel rolled down a 50' embankment. Almost took the truck with it. We left tire marks inches from the edge of the embankment. Had we been a class C or an A we would have been history.
It was a breezy but not especially windy day. Highway warning signs were not lit & there were plenty of big rigs on the highway before & after.
Wrong place. Wrong time for us. Huge sustainded gust came down the valley & hit us broadside. My estimation a 60-70mph gust. Dirt & gravel was in the air traveling horizontally.
These days we are very observant & very concious of wind speed & direction, especially traveling over open ground or have very high or open bridges to cross.
Forecast wind speed at under 30 gusting a little higher is not much call for concern. Higher, we are watching out for flags, tree movement, tall grass movement, wavelets on ponds, anything to give an idea of what is happening.