the number of times i have seen everyone rush toward the gate when the groundstaff announces the preboarding is mindboggling. even worse, the "out of order" pax are processed anyway.
Because as you noticed, it is a big scrum of people and it is just quicker and easier to process "out of order" pax, as it then causes even more congestion trying to turn them back against all the other half wits that cannot simply sit and wait for their row to be called.
I don't understand the mentality when you have allocated seats. You have a seat, you know what it is, you are at the gate, the plane isn't going to leave without you, so wait your turn !!.
When I worked with Lufty, they were trialling a zonal boarding procedure. Windows were in Zone A, Middle seats in Zone B and Aisles were in Zone C. The theory being that everyone in windows would go on first, followed by those in middle seats and those on the aisle would board last. They claimed to be able to board a full 747 in 15 minutes if everyone followed it correctly (we only had 737's, A320's and CRJ's at our station though). Not sure whether they kept it going though.