Is it "normal" or even legal for a "support car" to be driving along at (guessing) 15-20 mph in the shoulder on a US highway?
It seems like the slowly moving vehicle would be the first thing the truck driver saw, and if he misjudged that for a car in a lane driving at 75mph, I can see how this might have happened too quickly to correct for an initial mistake based on an unanticipated scenario.