These could be possible reasons why he would make a lateral move.
1) Iowa's DC resigned. So leaving gives him a choice as to who he will work with.
2) Pelini is known as a great defensive coach. Chance to build knowledge.
3) Nebraska has a great rep.
4) Similar to 2. Taking more than 1 job is good for career growth.
He is a guy that recruits Florida so that would be nice for the Huskers.
That may be the thing we are all missing here. Sure we need to fill a defensive position but we also need someone who can bring in some players. Even if the hire isn't the most experienced as a coach, if he is a hot recruiter, that could get him the job.
He hasn't exactly reeled in the recruits while at Iowa. Granted he's coached some very good DE's, I just don't get the hire.