This is a feature not a bug, the system is working properly. They fill up the lower leagues one league at a time because it is more fun to play with people than be in a bot league. When teams demote they go to a random league, this stops people being able to manipulate where they go. One thing that should be improved is new teams should be added 1 to the big 8 1 to the great 8 not the first 8 in a row to the big 8, that is silly.