Something like perhaps he believes the church needs leadership that he cannot provide at his age and with his health. Perhaps he see this as the best thing for the church at this moment.
I tend to agree. The world is entering a period of great crisis, and the Church will need to be able to stand firmly visibly for the Kingdom of God. I believe Benedict knows the Church needs vigorous leadership, more vigorous than a man of his years can provide. It is unprecedented, to be sure, but so is the size of the world's population and the percentage of that population who wish only evil on the Church.
Time to start praying that his successor be at least as bold in leadership and as pious in his personal life as this great "Rottweiler of God."