It's connected to the psychological isolation we feel inside our cars.
They become our own little world, and some of us feel more aggressive about potential space infringement than others.
Add to that, the fact that you can rant and rave from the isolation of your car, and no-one can hear you, which encourages you to do it more. Rest assured, if the other driver could hear you, and you hear him, people would be a lot less keen to sound off.
A few summers ago, i was sitting in queuing traffic for about twenty minutes - everyone had windows down because of the heat. A skinny youth in an ancient Capri at the kerbside and trying to get into the traffice queue, shouted to the driver of the car at his side "Hey, are you going to move that heap of s*+t!"
The driver got out of his car, and when fullly visible, as opposed to sitting down, he turned out to he about six foot tall, and built like a Marine. He strolled over to the Capri, leaned down until his face was leve with the skinny youth with the big mouth and quiety said "When the queue moves, I will move, and when I move, you can join the queue and until then you are going to sit there and be quiet. OK?"
The youth agreed with his voice trembling, the large guy got back in his car, and we all resumed queuing.
The moral? The other guy you are shouting at may be a lot bigger that you - so don't shout at strangers, it's not a good move!