But those 8 wheelers that were sucked up are around 6 tons (12,000 - 13,000 lb) each - empty - and several of them that they showed in videos looked like they were at least a few hundred feet up in the air. They're about 400 sq feet of pure sail (per side).
With a normal 24,000+ pound load on, the trailers stay on the ground pretty good, and I've never heard of a tractor being blown over; but when they're "dead-heading" empty it doesn't take a tornado to roll the trailer off the road - and when they go they can drag the tractor into the ditch along with them.
Out here in the flatlands, if a long-haul trucker is speeding, he's probably dead-headin' for home, empty; and the driver doesn't know much more than you do about which way that empty sail he's towing' is gonna blow. I let 'em past as quick as possible, and maintain distance.
Some of the "Schneider eggs" people talk about may be brown ones instead of the usual orange, when they all get back to that yard that got hit and see what happened.