We have had the same experience as RacetoSpaceMountain.
Before the kids were school aged, we always went in late September. Other than a few love bugs and the humidity, this was an ideal time for us to go because I hate standing in line for anything for more than 20 minutes. At least at that time, about 4-5 years ago, no line was that long except for maybe Soarin'...I think it was a long wait all day so we did that right at the end of the night and had to wait 10 minutes or so. I believe Toy Story Mania had just opened as well, so the lines for it were crazy, but only about 15 minutes during EMH.
Since the kids are older we go in early October which was very similar in wait times....right up until about 3 years ago. I personally don't think the crowd calendars have been properly adjusted. The first two weeks in October have been brutal the last two times we've gone (I think 2011 and 2012). But keep in mind this is coming from someone who enjoys the light crowds and can barely stand the thought of being in Disney in the summer with all those people, so a summer Disney goer might still find October crowds to be "light."
What has been happening is that more schools across the country are going to a "balanced" schedule...not quite year round, but close. The idea is that you have four equal grading periods and a week or two break between each one.
School schedules differ, obviously, but most go about 180 days a year. Our school wants to end before Memorial Day, so backing out of that always puts our Fall Break at the first two full weeks of October. You throw in Columbus Day and suddenly you have large crowds and I would bet the first full week is always worse than the second.
I think the other big issue with crowd levels in the first two weeks of October is the weather. On the two years where it was jam packed full of people, the weather forecast leading up to that week was extremely favorable (no chance of rain, high heat, etc.). I think that a lot locals and near locals saw that the weather was going to be nice, the kids were out of school, so why not head over to the Magic Kingdom for the day? There's nothing you can do about the weather though, but if you have the option of going when most kids are in school, do it.
Oh, and I should add this unexpected abundance of people was generally seen more at Magic Kingdom than the other parks.