I'm inclined to think that they saw enough, figured that somebody would pick him way before they would suggest, and left for that reason. The supplemental draft is a crapshoot, and it worked out very nicely for us.
The biggest reason I think this is because Saban has to know leaving early will start to become a tell if they keep doing it every time there really is interest. The other reason is that if they really loved the kid, waiting until the 5th wasn't really the way to make sure they got him. If Nick felt he was worth a 3rd and that there was a chance he'd get taken, he would have had to take him then. If you disagree with that, tell me why he took him in the 5th instead of the 6th, 7th, or just signing him as a free agent.