I had a conversation with Spinks and Suicidal Zebra via Twitter about the release possibilities a few weeks ago. I wonder if Blizzard felt they had to get the expansion out with non-zero time remaining in the annual passes of players (like myself) who signed up when the thing was first offered. They never committed to doing so, but having a month of pre-expansion launch event and a month after the expansion arrives within that one-year window is a bit of a difference, since many of us would have paid for that time anyway. Then again, perhaps the portion of the populace who are not annual pass subscribers - most likely the majority despite the surprisingly large number of annual subscribers - are the biggest flight risk.
Other than my lack of interest in Guild Wars 2, I suppose I'd be the kind of relatively inactive annual pass player that they might be targeting with this launch window. I don't know that I would have changed plans based on the date, but I'll definitely to see how my characters fare with the new talent system. As long as I have some Cataclysm-era stuff to wrap up anyway, there's no reason not to wait and combine that with test driving the new systems.