I wouldn't read anything into it. There are a lot of things to think about... it's his future for the next few years.
He's also, remember, going from king of the jungle to assistant king, defense. That is, I'd guess, a hard enough transistion by itself.
And while there are a ton of positives to being OC here... it's not like Saban is an easy-going back-slapper of a boss. It's a serious place with a workaholic leader. That might give someone who was just in charge pause. And it's pefectly legitimate.
Give him some time... better that he has weighed his options and decided this is exactly where he wants to be.