I had a feeling this would happen. Disney done the same with Monsters University moving it from November to the following summer, due to how well their films have been doing in the summer. Though i hope we get a better film for it.
I think the delay of The Good Dinosaur was the real reason plus the fact that Pixar films do better in Summer. 2014 will be the first year that Pixar haven't had a film in one year.