In more than one other sub-thread here, people have offered variants on the theory that some drivers are hostile toward cyclists and/or more likely to notice cyclist transgressions because those cyclists constitute an out-group and that's what we always do with out-groups. "Treating them differently just because they choose to live a bit differently" is the most recent version I saw.

Well, that cuts both ways. A certain subset of cyclists treat anyone in a car, or indeed not on a bike, as an out-group. They notice transgressions more, deny the out-group's right to do what they're doing, etc. It's not activism. Positive change is not the goal. It's just venting their own unresolved anger and frustration. If the putative cause of their complaint disappeared, they'd miss it ... and then they'd find a new "cause" to be anti-social about.