Drivers get angry when their traffic emotions are challenged beyond the coping point.
Since traffic emotions education is not part of the driver's training procedure, most drivers now have a low frustration tolerance and whenver that line is crossed, they react with anger. One such challenging situation is having to share the road with legitimate others. Most drivers have not trained themselves to become aware of their intolerance. Their parents showed intolerance for years as they were driven around, and years of reinforcement through TV scenes and commercials in which hostile and intolerant attitudes are portrayed as going on without bad consequences. In summary, then, drivers get mad at cyclists because the drivers have not trained themselves to accept the slowdown due to the cyclists sharing the road.