I remember a SI article from a couple years ago that spelled this out. After Missouri, Nebraska, and Colorado bolt from the conference, the Big 12 fractures with everybody south of Kansas going to the PAC and the rest go to the Big 10. Seeing how both basically expanded to like 16 teams, the SEC starts offering ACC and Big East schools out, and they get broken up, split between the SEC and the more northeastern teams going to the Big 10. So the PAC decides to cherry pick top MWC and WAC schools to even itself out. Now that three conferences control the top 60-70 schools in the nation, they tell the NCAA to fuck off and they make their own thing.