Most everyone while driving have at one time or another seen someone just totally lose it on the road, in which the driver perhaps rolled down their window to unleash a torrent of the most vindictive obscenities, or raised their fist in anger, or perhaps lifted up their middle finger in that notorious symbol of utter contempt, or even worse. There are, in fact, some drivers that if they perceive that you have cut them off, or done something or other that triggers their wrath, will proceed to stalk your car down the road, perhaps even weaving in and out of traffic just to confront you, come side-to-side with you, or suddenly maneuver their car right in front of yours, so that you have to slam on the brakes.
The thing about driving is besides the fact that the vehicle that you are driving weighs on average about 4,000 pounds is that in any reasonably large city, once you get behind the wheel and outside of your place of residence, pretty much you as well as the other people that are driving around you are effectively, anonymous. That is to say, you really don't know who is driving the car in front of you, to your side, behind you, and so forth, and you really typically don't actually care. Additionally, there are the rules of the road, implicit and explicit, that you as a driver may feel must be obeyed, whereas someone else's interpretation may be diametrically quite different from yours and this can easily cause conflict, given the right (or wrong) circumstances. Also, of course, there is the fear factor contained within driving, the actual knowledge that each and every day that people die or are seriously injured while driving, so that when you are cut off on the road, that fight or flight survival response is activated. So too, there are people that step behind the wheel that are impatient to get to their destination, or have had a bad day, or need medication, or are lacking in good judgment, to which all of these may be valid contributing factors to their road skills or lack thereof. That said, though, by far the biggest factor of road anger of them all, is the fact that you are unnamed, unknown, unknowable, and thereby effectively entitled to the most outrageous actions possible, since no one will ever be the wiser to them.
The thing about driving which is totally different than something like walking is that when you accidently bump into someone in the street, you typically don't overreact, but normally apologies are offered by both sides, because you can see one another as fellow humans; so too, it's why people don't get especially bent out of shape when someone cuts into the line or appears to do so, at a given store, because you may not be sure for a certainty that what you believe has occurred, actually did occur and even if fairly certain, your reaction typically ranges from perhaps a few choice words, often muttered, or perhaps a polite and fair inquiry about what you saw, but almost never do you begin with absolute rage. The difference between people to people, as opposed to car to car, is that there are societal expectations and rules that prevents us from going at each other's throat, especially when we really don’t know the other person, whereas inside our 4,000 pound behemoth of a car, we feel invincible, inviolable, never to be crossed, and ready to unleash our inner beast, knowing that if push comes to shove, that we can just drive ourselves out of trouble.
In short, road rage occurs because the circumstances of anonymity, the temporariness of the situation, and our ability to speed off into the night, makes it rather easy to have our say, to demonstratively display contempt, and feel triumphant in a certain, twisted way.