IIRC they had been on a kind of "probation" period with the AAU for at least a couple years, where they were acknowledged to not be meeting the requirements but were given a grace period to get them met, and didn't. Forcing the membership vote by the AAU committee responsible for such things, shortly after joining the Big 10. Nebraska knew they were not going to be an AAU member going forward, the AAU knew it, and the Big 10 knew it as well. Didn't matter. And since, Nebraska has not even tried to rebuild their AAU profile to regain membership, I'm told. AAU and academics is important to the Big 10, but not as important as they make it seem.