Why they are bullies could fill volumes, but why they pick on cyclists is pretty easy. We're easy targets.
From the few encounters I've had over the many years I've been riding, I also have basic anecdotal evidence. If you catch up with them, especially if they're out of thier cars, they are no longer bullies, but are generally afraid. At least in my case, I'm a large man who looks a little like Charles Manson and when the tables are turned (with me having a perceived advantage in size and/or lack of sanity) they have always become meek and apologetic.
It's basic animalistic behaviour. Might makes right.
Az