Several "triggers," fired by insignificant occasions, almost always instigate pure anger stupidly.
Right-click and wait. I believe that computers these days should operate more quickly than they do, and in any case that their operations ought to take place with a consistency. A productivity tool that spends random periods of time completing tasks might not be aggitating to everybody, but that's only because most people have learned complacency. And that makes the phenomenon even more outrageous.
Drivers who are overly-polite and insist on bestowing their right-of-way upon a pedestrian. It's not polite, nor kind, and in fact sometimes it feels dirty. I want to curse, or insult somebody, but it's quite rude to do so in such a moment.
People who don't pay attention to physical space and movement when walking in public. Women who are shopping do not care about you.