I think it's the fight or flight response people inherently have.
Having someone almost kill me might make the risky attempt to knock a mirror off justifiable.
but then again who knows, talking about a situation is very different from being in a situation.
I once caved in someone's passenger side door with a steel shanked stovepipe boot while trying to push away from a car that was about to sideswipe me. THAT is a fight-or-flight reaction. Taking someone's mirror off is a decision.
To the OP: This sort of thing is why I ride with cameras and why so many people are resorting to dashcams in their cars. While slowing to impede someone's progress is a potential charge under HTA 172 (aka "the racing and stunting law") you obviously completed your pass, and so were likely still moving at more than the limit. It's not up to someone else to decide how much you should break the law by, so you have my sympathy. It happens to us too, though:
[video=youtube;04aOukFLNGI]https://www.youtube.com/watch?v=04aOukFLNGI[/video]