What does driving around like idiots clogging up roads have to do with that?
I think it is an act of defiance.
At work, unless I'm with someone I know pretty well, no one really feels safe being a conservative voice. The only people feeling comfortable enough to voice their opinions are the liberals.
So I think being able to take a stand is unusual for them. And doing it in large numbers mean that they won't get a big target put on their back by their respective bosses, hr reps, or crazy coworkers.