Have a read of barrier aggression in dogs?

TLDR: Dogs use body language. smell etc to gauge wether another dog is friend or foe. When a barrier (like a fence etc) stops this assessment, they go straight to aggression as a defence/protection measure.

My guess is it a similar thing in humans.

We are stripped of our usual communication and threat assessment of the other person we would have in a normal encounter. We can't see their face, their body language etc etc. So if someone is prone to anxiety and/or aggression (two sides of the same coin), they jump straight to "That driver is a cunt who did that on purpose and to spite me".