Thanks for the input guys. Urban, that's a good point, it could have been one of the cars behind the front car that honked.
I am inclined to just pull over next time this happens. However, with no shoulder it would have meant either riding off into dirt and gravel, or stopping in the road, which feels uncomfortable.
I actually didn't consider that some people take the double-yellow line so seriously, guess I can't criticize that type of driver.