Randy was in a contract dispute, so they set up the interim belt between Nog and Sylvia, which Nog won.
Randy was then still in the contract dispute months later, so they tentatively set up Nog to defend his title against Mir, who was coming off the Lesnar win, after they would coach TUF together.
In the middle of TUF Randy reached a deal with the UFC though, which included a fight with Brock, even though IIRC Nog and Mir hadn't yet actually signed to fight on their specific fight date (though they obviously were set to fight), and Nog was interim champ, and Mir had beat Brock.
Essentially, they wanted to have 3 massive fights: Randy/Brock, Mir/Nog, and hopefully Brock/Mir II if both won.
The other factor was that they also wanted UFC 92 to be absolutely massive, and wanted 3 title fights on it. If Randy was returning in November and UFC 92 was in December, then Mir/Nog needed to remain for the interim belt.
It was a rather hilarious, but extremely exciting, time to be a fan.