This is a good example of confirmation bias. We see one Prius driver acting badly, and then when we notice another bad Prius driver, we start to build a story that Prius drivers are just bad. And then when we do that, we tend to forget or ignore all the other drivers of other makes behaving poorly, so eventually it just becomes all Prius, all the time.
Not saying that I’m immune to this. To me, Subaru drivers are the worst ever ;)