It's slightly different from New York, where getting in someone's way is a cardinal sin. As dirque once put it, "Just don't waste my time," is the rule in New York.
Maybe it's the in-person analogy to driving habits in the two cities: in New York, most of the obnoxious driving behavior seems to be from someone trying to getting ahead. In Boston, it often looks like random spite.
It probably also helps that middle-aged white men are the "default person" here, so what they do sets the image and model for the place.