The Obama admin says this has been in plans for over a year, however, I would have to assume the attention with the Rand Paul filibuster, as well as the recent circuit court decision against the CIA withholding documents relating to the program has caused this to become more a priority.
DB link
The WashPo article says a "secret report" (not so secret anymore) says the drone program has taken attention from the CIA's primary role of intelligence gathering, thereby causing blind spots in our intelligence and security.
WashPo Link
Admin officials claim this move will result in more transparency and also be better defined legally.
I find that laughable, considering Obama admin's history of the complete opposite. Contrary to Senator Obama's promises of transparency and openness during the beginning of his unending campaign, this admin has refused any kind of transparency and has fought almost any and every FOIA request relating to DHS, Drones, Homeland surveillance, etc.
Anyone really think this shift from CIA to DD will result in better transparency or legal definition? I think it's smoke and mirrors.