I haven't lived in Boston for a while, only did so for a short time, and never bicycled there, but I believe you. It was in Boston I learned the "Boston left turn", where you stick your car out into the nearest lane of traffic, blocking them, until you can make your turn. Terribly rude, and I broke myself of it soon after leaving, but sometimes the only way to make a left turn in that town, "divide and conquer".
So in addition to all the other reasons given, I imagine that the rudeness of the drivers to each other in that town also makes them more likely to take it out on someone they perceive as weaker or having less right to the road.