I should have put money on it...
We keep hearing that Harrison Ford's injury has left the production scrambling. First, the shooting schedule had to be changed. Then, we heard that the film was looking at changing the script and enlarging Oscar Isaacs role to compensate. Disney denied that. And now the latest rumor is that J.J. Abrams asked to push the release date back to May 4, 2016. Latino Review doesn't directly pin Abrams' request to the break (they instead say that Abrams "thinks that his Star Wars should be released in May like all the other Star Wars movies have been."), but it would still make sense, if the break is anywhere near as bad as the rumors have been saying.
The Latino Review article further states that Kathleen Kennedy of Lucasfilm agrees with Abrams, but it's Bob Iger and Disney who are determined to get this film out in December 2015. As always, beware of stories quoting unnamed sources. [Latino Review via Slashfilm)