My two cents on why they would leave their current gig:
1. Grimes-While Baylor has been very solid the past few years, they are also part of the Big 12 which I believe is a conference on the decline. Thus, getting out sooner than later might be appealing.
2. Lutwig-Utah has been a very solid team under Whittingham but the Pac 12 tends not to get as much attention as the SEC or BIG. Plus he had huge success in the BIG and his Wisconsin offense in 2013 set records. Maybe he feels he could replicate that at Nebraska?
3. Ruggerio-Wake has had a good run the past few years but cannot match the history and support that Nebraska offers. I also think the BIG would be a step up from the ACC.