What's all the aggression about?

There was a car overtaking me on the left. My arm hit the mirror and turned it in a bit [...] Anyway, I apologized, said it was my fault.

The thing you did wrong was not apologise in the first place. It's the same as walking into someone and not apologising - why would you do it? Yes, no harm done, but IMO you lack manners as well as the car driver. The fact you (seemingly) ignored your misdemeanour in the first is probably what wound him up.

Hang just a cotton-picking minute. The car hit punkpixel and you see this as a misdemeanour on his part? The only thing he's done wrong is apologise for the fact his arm was in the way of an overtaking car.

You got it wrong cowboy...