Statistical probability.
I cannot believe that a large mass of poeple would change countries to maintain this imnbalance on purpose .I think 90% of the poeple here agree with you when you say that inbalance IS boring (me included). For all I know a majority of poeple stick to their country for long periods of time (and even if the MOL and the AK's are potent squads, I do not believe they are resonsible).
As we have all seen, the "drop in numbers" will flow from one country to another, I think it might just be ... KARMA
PS: I have another theory and it's got to do with StSanta's Arse being responsible for the migrations, well, more it's mass, realy, but I need to do some more research before I can publish anything