I've often wondered why drivers get disproportionately angry at cyclists for "getting in their way" and "slowing them down", but rarely abuse other drivers for getting in their way and slowing them down far more.
I don't think motorists get angrier at cyclists only that cyclists are more vulnerable to abuse. It's more difficult to intimidate someone driving another motor vehicle without risking your own vehicle. Parking up and leaping out, intending to yell and threaten violence doesn't really work on other motorists. The other guy just has to wind up his window and keep on driving.
When it comes to complaining about other road users, cyclists aren't the only easy target. Almost everyone I know has a bit of a moan about getting stuck behind a tractor, most of them ruefully accept it as a fact of life, a few don't mind it at all and plan their journeys with a little extra time just in case, and a very small minority get wound up and rage. None of these people would dream of close passing a tractor or brake checking them, or throwing something at them.
I've heard people complain about a driver sticking to just below the speed limit far more often than I've heard people complain of cyclists (and this before I started cycling regularly).