I would imagine that just about everyone has had that disproportionate reaction to something when they're at the end of their rope. ONE MORE THING HAPPENS TODAY AND POW! SOMEBODY'S GOING TO GET IT!
Then someone comes along and says/does something that would normally have you just shaking your head and rolling your eyes and you can't deal.with.it.right.now!
That's what scares me about "road rage". I accidently pull out in front of somebody or cut them off and they've had a bad day and that's the thing that's going to push them over the edge. What would normally be a honk and a middle finger raised and someone yelling, "LEARN TO DRIVE, YOU *&^#$@*&%?" often turns into a gun pulled out from under the seat and a tragedy.