I see drivers doing the same thing with motorcyclists who are legally filtering when traffic is at a standstill. Really makes no sense since those motorcycles will be completely out of the way of the car drivers trying to block them. Same with bicyclists in a bike lane - traffic in the regular traffic lanes and in the bike lane move independently of each other with neither one holding up the other. Except for someone deliberately blocking the lane as in your case.

Last summer we were returning from the solar eclipse and traffic was barely moving. We were in the right lane (of two) on the interstate when I saw a motorcyclist coming up from behind. I inched a bit to the right to give him more room, but the car driver to our left also moved right in a deliberate blocking attempt. After honking for awhile, the motorcyclist reached back and drew out a handgun which he started waving at the car driver. Fortunately the driver backed off and the situation was resolved. Certainly don't condone the brandishing of a gun - but the deliberate blocking attempt was also uncalled for.