Nikosandros
Golden Procrastinator
Since it looks like the PHBs, MMs, and such are all coming at about the same time each year it would make sense that the Dragon Annuals will come out around August then for each year. Plus an easy book around Gen Con isn't a bad idea.
Well, the first PHB, DMG and MM came out at a different time (since it was new edition launch). There really isn't any good reason to be forced to put out 'serialized' books at the same time each year.
The second motivation sounds far more likely, though.