Perhaps he sees a greater likelihood of a judicial dismissal with this judge, which lets him off the hook to some extent, coupled with the fact that he did get the indictments, especially if Mangum recants at an opportune time or announces through counsel that she will not testify due to emotional distress or mental illness.
But most likely there is something going on behind the scenes relating to your question that we are simply unaware of right now.
http://www.webcommentary.com/asp/ShowArticle.asp?id=gaynorm&date=061023
Best Duke Case Defense: The Three's Obvious Innocence
Michael J. Gaynor