For several seasons (from level 4), I was in the same league as 4 of my friends. Then at the end of one season, we all got put into separate leagues. It lasted just 1 season, then we were back together again. Last season, I was again separated from them, but once again, this time I'm back with 2 of them. So I think its just to give a bit of variety to the game. It has nothing to do with servers.