Game theory. They haven't called boarding yet. Just someone decided to queue so everybody decided to queue. Except I have no idea of any advantage from boarding before others apart from overhead compartment space. But flight attendant already took a bunch of trolleys from us to check in.

It's the humility/risk problem. As long as you've understood better than everybody else, you can get comfortable on a boarding seat and wait to get on the plane. But given the risk that you'll miss your flight, you'd have to be really sure.