I think the delay of The Good Dinosaur was the real reason plus the fact that Pixar films do better in Summer.
Yeah they removed the director, so far into the project, so more time was needed i guess.
2014 will be the first year that Pixar haven't had a film in one year.
Not quite, this last happened in 2005, and was much like this in the early years were they'd release a film every other year (3 years between Toy Story and A Bug's Life!)
But it'll feel more like an event again when the next one comes out. It'll be worth the wait though as it'll be the first time two Pixar films are being released in the same year.