I’ve found this behavior to be more or less common in different places. It’s extremely common in Florida, for one. It’s pretty common in some parts of New York too. By contrast, just one state over, I haven’t encountered it in Connecticut at all (though I haven’t driven there as much as I have in Florida or New York, and I’ve mostly used the highways, where slower drivers are explicitly instructed to stay in the right lane. So there could be a sampling bias.)

I never understood it. It’s like the driver thinks that if somebody passes them, it’s a personal insult. It reminds me of those who speed up when they see that somebody might merge in front of them; like they feel possessive of the public road somehow.