The seats are assigned. People have been standing in line for 15 minutes now. Why on earth would anyone want to stand there, when they could just sit and wait until the line clears?
I understand wanting to get off a plane ASAP, but boarding? You just end up sitting on the plane, waiting for everyone else to get on.
Because airlines have hiked the cost of checked luggage to absurd levels so a large number of people have roller suitcase carryons. I have a small messenger bag I travel with that's soft-sided and I want to get on early so I can make sure it's over my head in case some asshole tries to cram their bag in over mine.