WoW is ready to launch Cataclysm. Many, many old WoW players have dropped their current MMO's and have been power leveling their WoW characters for the last month and a half. Blizzard is really changing it up, it's not just another top-level endgame expansion from what I've heard... it's a major mechanics change. So it's hard to say how long it will keep other MMO pop's down, but it IS happening with pretty much all of them right now.
I'm not saying any of the other theories are wrong, but WoW is the major factor right now, and SE just doesn't have enough content to hold people to this game yet.
The other main reasons I've seen people leave are either because they can't ever find Gear to purchase, or because there doesn't really seem to be anything to do other than grind or craft.
I used to play wow and I like the idea of cata but i'm having fun in FFXI so I havent even bothered to re-sub to wow yet and I surely would go nowhere after a game update on my current game if it was decent. If you are having fun in your current MMO you won't run back to wow and if you do(even worse after an update) the chances you come back are pretty minimal, especially if it's so new and you don't have much time invested.