It's considerate communication. Lurching into the next lane .08 seconds after the blinker first flashes says things like "Your life isn't worth the basic consideration and respect of communicating my intentions" and scales up to "I'll communicate, but you're not worth any sort of common courtesy" - that can be upsetting to people.

It doesn't even have to be real. There's huge room for miscommunication. Unpredictable movements and perceived aggression, or unwillingness to be considerate to other drivers on the road, there's a whole wealth of information being processed, regardless of how little is actually real.

Now add the total lack of accountability for the driver's emotional state (don't you love yelling at other drivers, completely free of judgement?), and you can see how things spiral into road rage so relatively easily, even if everyone involved is normally a pretty chill, rational person.

If you're tailgating or brake-checking, or being inattentive and sloppy, you're basically threatening people's lives with a few tons of high speed metal, even if you don't intend that at all.

Ideally, the rules of the road are meant to reinforce a mutual understanding of the game being played. Behavior occurring when expected, proper signaling, observing limits, and making the effort to communicate where possible is a signal that you and the other driver are both operating by the same set of rules, giving you both confidence that neither of you are going to be a danger.

I've seen little "cute" exceptions where locals develop a subculture of dangerous assumptions and then get aggravated when someone from out of town doesn't immediately get it. There are other areas where aggression and what amounts to flagrant disrespect are the norm, so you've always gotta be adaptive, but ideally you get people conspicuously following the same set of rules as a sort of game theoretic optimal strategy for driving.