I think you all are being much too generous.

My opinion, some people are just bullies. Being in a car amplifies that - for the same reason that a lot of people feel comfortable picking their noses or scratching their crotches in the car, or trolling on the internet. There's a feeling of being in a bubble, of anonymity and safety, even if that isn't exactly true. So you get everyone who's inclined to roadrage at other motor vehicles (a big population right there), PLUS those who just avoid that kind of road rage because they understand that taking on someone else in a motorized vehicle might be biting off more than they can chew. A cyclist or runner isn't likely to be able to retaliate and put the bully's life or safety, or their passengers', at risk.

Now, that said, when someone buzzes me without explicit signs that it was intentional (yelling, throwing something, honking their horn), I'm more inclined to think they target fixated, and buzzed me because they don't have a clue how to steer a vehicle with wheels, rather than because they intended to scare me. Now that I'm running much more than cycling, I get to face traffic and watch drivers aim their cars at me all the time, like as many as one vehicle in five on a straight piece of roadway where we have plenty of time to see each other - just because they never learned about target fixation in driver's ed. Most of them correct before I start thinking about diving for the ditch - those are the ones cyclists don't even realize how close they came to being flattened.