The impetus?
The Big12 immediately realized that the ACC Network & possibly more importantly expanding the GOR made them the weakest link.
They had to do something, otherwise it would just be a matter of time before they were picked apart by the others. The question is, does adding the like of Cinncinati, BYU, etc. fundamentally change the perception of being the weakest link? I seriously question that, but their contract at least allows them to make good $$ for the programs until such time as they break up (assuming they won't be giving any new additions a full share immediately, it is more $ in the pockets of existing programs)