I'll give you some insight from a person who has been on the non-cyclist end of such things, me.
Yup, before I decided to give cycling a try I was your average hillbilly who couldn't hid my red neck if I tried. Granted, I'm still a bit of a redneck, but in a less of an assholish way.
For me, it was all about getting my jollies at someone elses expense, and cyclists are easy targets. They are for the most part skinny little fella's with no threatening muscles to show. They made a very easy target since I knew that if it came to physical encounters I would be the one walking away from the fight.
Me and some buds would be riding around, doing god knows what, and we'd see this skinny little brightly colored guy up ahead. He looked odd, like a clown, and out of place. Showing off his sausage and hard boiled eggs through his little tight girly shorts. So why not yell something out the window or toss my wad of chew at him. We'd laugh and drive off making jokes about him and what he and his buddies would do later once they all got sweaty and wot-not. We look in the mirror and see him stop and try to wipe the tobacco juice from his clothing. It was pur sadistic fun that we could have and be totally anonymous about it.

I'm no longer that person though and have paid my dues for my past life. But I'm not going to put on a false face on here either just to try and fit in. I'm am who I am and not who I was.
But that is why we would do it.