It's a combination of everything the previous posters have mentioned. We've gone to WDW around this time frame for years, as it's DD's bday on the 21st. It usually aligns with a school fall break. It used to be a great time to go, but it has slowly evolved into a very busy season.
Last year was one of the busiest/craziest ever! We had to spend one night at Pop before our ressie at the Poly. We could not find availability for that one night, and my TA watched it for months.
Good thing she turns 21 this year...we are headed to Las Vegas instead.
ETA: A clarification to a PP.... Hurricane season is still in full swing in October. We were at WDW in 2005 when Wilma made landfall. Hurricane season runs through the end of November.