Word on the street is that Apple is behind schedule on iOS 7 due to the management shakeup that occurred when Scott Forstall was ousted from the company. Jony Ive has taken over as head of hardware and software design, and with that will come sweeping design changes within iOS. Daring Fireball's John Gruber has gone so far as to say that he's heard that Apple has had to pull engineers off os OS X 10.9 in order to help get iOS 7 ready for release.
It may sound odd, but Apple actually took a similar strategy back in 200, pulling developers off of OS X 10.5 Leopard in order to ensure that the iPhone would be ready on time, resulting in a four-month delay for the desktop OS. Of course, Apple had a lot less software engineers working on iPhone software back then, so it's pretty telling that with the expansive growth, there are still challenges getting iOS 7 out of door.