The mystery has been solved, or at least clarified by at least by one of the Big 12 schools in question. According to ESPN, Missouri is likely to stay put in the Big 12, despite the swirling rumors that Missouri would join Texas A&M in joining the Southeastern Conference.
Missouri has been a part of the Big 12 since the conference began athletic competition in 1996. According to the Big 12 website, it is "the only major conference in the nation to determine its champions in all sports directly on the field." Colorado and Nebraska recently left the league, arriving in the Pac 12 and the Big Ten, respectively.
It should definitely be stated that there is still some uncertainty looking forward. Big 12 Interim Commissioner Chuck Neinas phrased it this way: "I think that once (the Missouri board of curators) have an opportunity to fully understand and comprehend what the conference is doing that they will agree that Missouri should continue to be a good member of the Big 12 Conference."
The underwriting seems to be that Missouri could still bail if the process does not unfold in their preferred direction. Neinas also admitted that the SEC had not actually contacted him or anyone else at Missouri about joining.
He likened the current relationships of conferences and schools to boys asking girls to the prom. The Big 12 is "the one who's tried and true and you know is going to be there."
It does seem clear that this quandary will not be resolved any time soon for the Big 12. Current discussions are taking place about schools to add if circumstances dictate such action. Neinas claims that plenty of schools are interested in Big 12 membership. He is no stranger to this scenario, as he was the Commissioner of the Big 8 conference, which joined with four Texas schools to create the Big 12.