The problem with that is that B12 seems to be in better shape because they have been working on it for a year. I am not sure what contracts have been signed, if any, but for BYU and others to bail within the first year would be odd. Not putting it past people, but it would be odd.
Remember the BigEast circus?
TCU "joined" the BigEast in 2010 and then quickly backed out and joined the B12 instead the following year because the BigEast was imploding. A little different situation here since none of the B12 announced additions (BYU, Cincy, Houston, UCF) would be asked to join the PAC and even if some B12 member schools got snapped up by the PAC, the new additions would likely not be able to find a better home than a weakened B12.