While the rumored move of Florida State from the ACC to the Big 12 is just that, a rumor, It could potentially set off a ripple effect leading to another ACC school jumping ship for the Big 12.
Clemson has long been rumored to be a potential target and moving partner for Florida State but the Tigers' ties to the ACC as a charter member run deep. Miami on the other hand doesn't have as much history with the conference and could try to chain itself to FSU and come along for the ride.
If the Big 12 could add both FSU and Miami it would give them two major programs in the state of Florida which would further cement the conference's status. The rumors make sense but the first step in the process is the existing 10 teams of the Big 12 deciding that expansion to 12 teams is the correct thing to do.