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.
Anxiety (especially now that planes are oversold and standby passengers are nearby waiting to grab empty seats…), the need for overhead bin space, not wanting to have to climb over people, illogical impatience, etc.