This is the cynic in me, but I think they had to do something to keep the markets happy. ESPN is in trouble which means trouble for Disney's stock price unless they can make up for that revenue somehow. The single most valuable, but yet under utilized, asset Disney has right now is Star Wars and they have to capitalize on it to the fullest extent possible as soon as possible, even if it means building faster than is typical for them. This is nothing but good news for fans of Star Wars and Disney theme parks, IMO. We're seeing it in the push for merchandise, we'll see it in December when The Force Awakens comes out, and we'll see it in the parks with the Star Wars lands.
On the other hand, the skeptical side of me says that "2016" will end up meaning like November or something. But I really think the trouble with ESPN is forcing Disney's hand with their Star Wars land concepts and we'll start seeing construction sooner rather than later.