You never know what the Pac12 is thinking. Maybe they are looking at their expansion candidates to get to 14. Maybe they are wondering about BYU. (I doubt it, but you never know.) What if the Big12 waits, and then the Pac-12 gets involved? I'd sign BYU as soon as I could.
I guess they could have already *signed* BYU, and are just waiting to announce it... Maybe that's what you're saying.