I think that the reason people are all leaving so soon, is because they have been wanting to for a long time. Its kinda like a good xp party. several people have to go soon, but they don't want to be the guy who breaks the chain, and you will frequently find people staying longer than they wanted to/ should have. Of course it can't last for ever, so when the first guy leaves 2-3 people go with him. The same applies here.
People come, make friends and lvl to 75 do sky etc. They get bored but keep playing because their friends are. WoW or another MMORPG comes out and one person decides to try it out (and also leave FFXI), so his friend follows his lead, and so do his friends, and so do their friends and so on and so forth.
Of course it could also have something to do with changes during the summer...such as many people may have left highschool, and work doesn't allow them enough time or various other reasons.
Hell maybe they were just waiting for an interesting MMO and WoW looked like the one.
Also just a note: you look at all the people leaving as a bad thing, but when was the last time the SE tried to solve MPK problems? or give us BLU which we have demanded for god knows how long, or any of the other stuff they used to just ignore? Oh yeah there wasn't...
Now call me suspicious, but i highly doubt that the large number of people leaving all at once and SE suddenly giving a shit are complete coincidences.
Just food for thought.