The way I understand it, the map resets and the base ownership is set to the next in rotation for that map. In other words, it's setting which bases are owned by which countries the next time that map is used.
Actually, the map resets to its default configuration. When a map is built, there are no "Bishop, Knight, Rook" fields. Everything is "Country 1, Country 2, Country 3." As you know from dot commands, Bishops are Country 1, Knights are Country 2, and Rooks are Country 3 on the client side.
When a map is first put up after a reset, the server randomizes which country is which on its end in order to give fair play over the long term. When that map is finally won by someone, the server has dropped that recognition and the map returns to the way it was built.
