https://www.warlight.net/blog/index.php/2015/01/update-3-3-multi-player-dashboard-map-of-the-week/In Short:
◾The highest rated map created within the last week
◾A random map that’s never been voted on before
◾The map that got the 2nd most votes in the previous week (or, on week 1, just a second random map)
◾Two maps that are nominated by members. Players can only nominate one of the two slots each week, and may not nominate a map they created.