Sometimes it's as simple as the fact that you had the nerve to honk, AT HIM! Don't you know who you're dealing with? HOW DARE YOU!
I agree, but I'm a realist...and there's always 2 sides to every story. Lots of Dashcam videos of road rage incidents out there that were brought on by the very person posting the video...because just before the beginning of the video they intentionally did something to the other person to incite it. Of course you never see that part, right? Just like the police beating videos where the person showing the video conveniently doesn't include the part 30 seconds beforehand where the offender punches an officer in the face and starts grabbing at his gun...all you see is the (rightly deserved) beatdown he gets after the fact.
But yes, some people are set off by nothing. I've had people give me the finger while passing me because I dare only drive 10 over the speed limit in my work truck. Heresy, I know.