They have become more crowded. The number of vehicles also has risen. People are busier, time is at more of a premium. So road congestion causes even more frustrations. These emotions lead to impaired judgment, saying or doing things they later regret, including engaging in risky behaviors, while attempting to punish or retaliate against the offending driver.
Cars offer control, isolation.
Cars offer the promise of retaliation without consequence. In a car we feel invincible. Cars remove the civility and humanity normally present in society. Cars give people a buffer zone, there is a sense of isolation afforded by a car. That isolation makes it easier for people to dehumanize other drivers, it makes it easier to be aggressive towards them.