Dave got demoted from OC after a couple really bad years of offensive production. I don't think he is really popular with the fanbase and D’Antonio's loyalty to him (he kept him at OC under great pressure for a long time) surely played a role in Mark stepping down. I would be shocked if he was named the new HC.
I think if you are going to resign, you do it during the season, or immediately after the season. Give the AD a chance to fill the position when there are candidates out there during the normal job cycle, give the recruits a chance to know who the new coach is.
Waiting until a few hours before the late signing period is not cool. Maybe he just found out he has a serious health issue, or his wife does. If it is something like that, I have no issues with this. If he waited to try and lock his class in so he can say he left the cupboard full, I don't have much sympathy.
I could see Pitt HC Pat Narduzzi heading west though.
Update:
Wow. Sounds like he was forced out in an attempt to minimize the penalties the NCAA is going to impose on the Spartans. What a sad ending to what was a really impressive coaching career.