I think the Big 12 wants to become "too big to fail." They've determined that there will be 3 power conferences and they are determined to be number 3. Maybe seeing their potential demise with the losses of Missou, Colorado, Texas, Oklahoma, Nebraska, and A&M allowed them to be more unified in going after the PAC and ACC.
The Big 12 is A (not THE) bad guy here. They helped blow up the PAC by taking enough schools that others had to follow or be left behind. They could do the same with the ACC.
I personally can't see UConn getting a full spot in the Big 12. I think they've been using us for years. But there may only be a handful of okay programs left in the ACC by the time the Big 12, Big 10 and SEC are done. I don't believe the Big 10 or SEC want to take on any ACC teams now, but they will.