It pretty much boils down to arrogance and the belief that your rights trump all others, on both sides.
There's a similar route in the Oakland hills where the spandex racer wanna-be's frequently ride two and three abreast. Every time someone gets splattered it's automatically blamed on the car driver. I had a conversation with someone who frequented the place and asked him that if I encountered a group of cyclists on a blind curve with an on-coming car at the same time, if I should have a head on collision to avoid the bikers. He just looked at me like the answer was obvious. Seemed pretty upset by my answer.
This behavior isn't relegated to the roads either. My wife and I do a 25-30 mile ride on the local bike trail once or twice a week. There's a couple of gaggles that come through two abreast regularly. One woman actually tried to push me aside. I've contemplated bringing a bit of broom handle to place smartly through the spokes.