While I don't think this is the root cause of the delay (the falcon wings and the second row seem plausible reasons to me), I do believe it likely that what Tesla released in the P85D and Auto-pilot is what the first generation of Model X was supposed to have drivetrain and assistant-systems-wise (at least ever since Model X got delayed beyond 2013). But since it didn't launch yet, I have speculated Tesla decided to wait for the second-generation of Auto-pilot for Model X, hence waiting for the mass availability of nVidia's Drive PX with its numerous all-around cameras. This speculation is based on seeing what looks like Drive PX
camera locations/lack of mirror on Model X mules, Drive PX release info and timing from nVidia and Elon Musk saying at Drive PX launch something along the lines of I'm excited to show what we've been working on together work the X.
This would allow them to launch the Model X with something Model S doesn't yet have and wow a little. Yet this also makes me a believer that if Model X launches with nVidia Drive PX, Model S will receive this update near the same time.
I have also speculated that if Model X launches with a redesigned nose, it is possible a Model S facelift with a new nose would be included as well, based on the September 2014 "leakster" whose CAD drawing fot pulled by Tesla claiming Model X nose cone is gone, and on the head designer of Model S reportedly saying in an interview the one thing he'd change on the Model S being nose cone and the timing mentioned in Tesla's shareholder letter of November 2014, saying "Our focus will be on scaling up Model S production over the coming year, so no major platform changes to the hardware are planned in the near term" which alludes to a year of no major changes, ending around autumn 2015. It seems plausible.
Of course, if this is what happens, judging by the latest 70D change, it is also possible Tesla is waiting for Model X launch to launch the 70D's sister battery of higher capacity with Model X, which would also trickle down to Model S, perhaps autumn 2015.
I am also in the camp that believes Model X reveal will happen as near actual sales as possible, to minimize its effect on Model S sales, assuming it will come with new features that might make Model S buyers think twice about the current Model S. Yet I don't think Tesla will want to reveal all the bits and bobs on Model S before Model X (and rely on second-row and Falcons to wow solely), so they are juggling these concerns at the moment I'm sure and being extra secretive. Hence, that's why we need to i-spy and speculate even harder, to fill that gap for the prospective buyer.