That's not true; drivers percieve everyone as a nuisance-- even other cars. The problem is that a driver is more likely to become physically aggressive with a cyclist than another car.
The difference here, though, is that there were enough cyclists around for there to be immediate consequence.

I've ridden many critical masses before. While i hate them and won't ride them again, i've been in enough to see the consistent trend of drivers with an unwarranted sense of entitlement creating unnecessary situations in order to do little more than prove a point.
The riders-- i'm certain of it, as this happens every time-- most certainly didn't begin circling the car for no reason. what most likely happened is, a motorist got pissed off because he didn't like what he was seeing, drove too close to a group of cyclists, yelled something, then most likely came into contact with one or more as he attempted to drive through the procession. This sort of behavior 100% of the time results in the driver being confronted. It's most likely at that point that the "witness" turned around to see what was happening, and "witnessed" a mob of angry cyclists surrounding the car. What she probably didn't see was the multiple cyclists the driver hit which escalated things to that level. No, the incident won't help anything, but you can't possibly forgive the actions of a driver willing to cause bodily damage to a cyclist (or anyone for that matter) because he's a little annoyed at what he's seeing.