maybe I'm imagining things, but the first division to fill up often has the highest-rated managers, especially early in the SOM "season" (March-May)
I suspect early in release of a 200x game the frequent players are stoked about the new cards and cannot wait to start teams and thus start leagues or jump in to anything forming.
I have seen the opposite tend to transpire in the ATG game. Most experienced guys there may wait to jump in so people do not have as much chance to jump out when they see really tough competition in the league. Plus I have come to the conclusion there are a few who like to be in most leagues but are too impatient to wait for leagues to fill. These people watch for leagues that are at least half full and then jump in. They can get by with this because the more static nature of the player pool plus their experience means they can usually throw a competitive team together within minutes. Always amused me how a league could sit stalled for days and then once there were 7 teams league fills within an hour.
I am too indecisive to throw a team together quickly so I tend to end up in the east as an early adopter of a league.