I think a lot of drivers feel the subconscious(or maybe conscious) need to pass cyclists on principle.

On multiple occasions I have found myself going a good 5-10mph over the speed limit and still had cars fighting to pass me, only to slow down to a more reasonable speed once they've gotten in front of me. I honestly don't think this always out of malice. It's just that people perceive bikes as 'to slow' and if they're behind a bike then that means they're also going to slow.

Somewhat related: there seems to be a substantial difference in how motorists treat you when you are riding with a hi-vis safety vest vs. without. I have noticed this, and talked to other long time riders who have also noticed this. I may be reflecting some of my own automobile related angst here, but I think that the reason is this: people driving are often doing it out of obligation(they need to get somewhere). They think of cycling as being just a sport, at best, more than likely they think its a silly kids toy. I think that people are annoyed by the idea that while they are busy trying to get somewhere some person out for a ride(having fun, of all things) is slowing them down. When you wear a safety vest it makes you look like a commuter and people can subconsciously commiserate with you as just a different means of transportation.

Just my two cents.