The last round of realignments absolutely crushed the Big 12. The monsters of the conference then and now are Oklahoma and Texas but consider what they actually lost. Nebraska, Missouri, Colorado, & Texas A&M. All in their own States heavyweights. All in their own States Flagship universities (Texas A&M shares resources evenly with Texas). All except Nebraska AAU members (and Nebraska had the designation for roughly 100 years.
The only reason The Big 12 survived was because Texas wanted it to. Oklahoma is a juggernaut and has multiple options but if we are honest, the league is held together by the whim of UT and everyone knows it.
The thing now is that the SEC and B1G are about to lap the Big 12, ACC, and Pac 12. My guess is that Texas and Oklahoma got together and said "the rest of the league is basically dead weight. If we stick together our value would probably improve the most by going to the SEC. The ONLY issue is A&M blocking but I think that objection can be overcome by all of the money that can be made. In the unlikely event that A&M is successful at blocking UT, I foresee Texas & Oklahoma refusing to re-up in the Big 12 essentially imploding it as the SWC was imploded and then shopping themselves as a couple to the Pac-12, B1G, and ACC or considering to go independent. If I had to bet I think order of likelihood of the preceding outcomes I'd say:
35% Join the SEC Makes to much sense (and money)
25% Keep the Big 12 alive
15% Join the B1G (Good money terrible Geography)
10% Join the ACC (Tobacco Road is intriguing, bad geography)
10% Go independent (football would be find but need somewhere to park other sports)
5% Join the PAC 12 (If they wanted it they would have joined circa 2010, bad kick times, bad geography, and terrible leadership has taken the luster off of the left coast.)