A lot of schools have fall breaks in October. Plus, the Food and Wine and Mickey's Not so Scary party is going on. Plus, the discounts end in September - so DVC studios would be even more of a bargain for point renters. Anytime there is a potential for a long weekend getaway (around a holiday) I've found that rooms are harder to book. The same thing goes for good prices on airfare to do a long weekend getaway if there's any kind of a holiday.
We have noticed since we joined almost 6 years ago that it is a lot more important to book at the 11 month window than what it used to be. Between all the internet sites and books that promote renting DVC points to get a bargain, not to mention the fact that DVC has grown a lot, means you have to book early for certain times of the year.