No idea on first class, except that I think a lot of your business travelers rush the desk to see about upgrades when the gate agents first arrive.
As for everyone else mobbing in, I used to wonder why too and then one day I was sitting on the plane and someone in front of me was queried by another passenger "what seat are you in?"..."blah, here's my ticket"..."I'm have the same seat assigned!".... dude next to me says "and THAT is why you get on the airplane as fast as possible, I've seen it happen before and first one sitting down wins".
Now, I dont know if that really happens very often, I would think the booking computers are smart enough that it doesn't, but it did that time, and according to the guy next to me it happens enough that that's why he rushes on.
Personally, I'd rather get on, get overhead space, and sit down rather than stand in the cattle herd in the isle while everyone panics and get's pissy trying to find space, pushing up and down the isle desperate for that last space for their precious carry on, while everyone else is staring at them wanting them to get out of the way.