I think FSU turned down the Big 12 and was turned down by the SEC and B1G. That left nothing for them but to strengthen the ACC. Same is probably true of Miami, and possibly Clemson.
The short term thinking around here is amazing. The ACC supposedly sucks. And at present, yes it is down. Miami has been imploding, UNC is a scandal factory, BC has multiple self inflicted wounds and is trying not to bleed to death and VT has just been sub-par. But the programs are there for it to be an excellent football conference. NC State is improving. All four of the others I mentioned will only get better from here. They added Notre Dame games for SOS and market visibility and the just added a Louisville team that is solid at the moment. Hell even Syracuse is better now than they have been in over a decade. Basketball...they are loaded. Baseball too.
If you take the long view, the ACC is much more attractive league than the Big 12. This has been the case throughought realignment. The ACC lost Maryland, and added BC, Miami, VT, Pitt, Cuse and Louisville. They are plus 5, not even counting Notre Dame. Maybe they have something to offer? Meanwhile, the Big 12 lost Colorado, Nebraska, A&M, and Missouri (if you go far enough back, Arkansas). Minus 2. They added WVU and TCU, one of which is a school they formerly rejected. It's a mess. It just happens to have a decent TV contract and the biggest program in the country. If a GOR gets challenged...it will be the Big 12's, not the ACC's.