Lots of good suggestions here, including the difficulty in communicating inherent to being in a car.

I think another important factor is that driving itself is stressful. Surveys of commuters consistently show that people who walk or cycle have the highest satisfaction with their commute, while motorists ranks somewhere from the middle to the bottom (i.e., either ahead of or behind people who use public transport), depending on the study.

When you put people in a stressful situation where it’s difficult to communicate, inevitably some people lose their temper.