One reason why it could be a bit more crowded is if Halloween Time has begun at DLR.
The date for the official opening day of the Halloween Time season (or any season at DLR) changes every year. Typically it has fallen on the next to last Friday in September, or even the last Friday in September in one case. This year, Halloween Time begins on Friday, September 16th. Whenever a season such as Halloween Time or the holidays first begins, people come out to see what's new or different and crowds pick up a bit.
Next year, Halloween Time would probably either begin on 9/14 (which almost seems too early to me) or 9/21. Either way, you will be there for at least part of the season. If it begins on 9/14, then the novelty of the opening weekend may have died down a bit by the time you arrive on or around 9/18 and the crowds could thin out a bit.
If the season begins on 9/21, however, you would probably have a couple of days of lower crowds at the onset of your trip and then people will start to show up before the weekend to be there when the Halloween season begins.
Also, a new land (CarsLand) and other new things in California Adventure will be opening/appearing next year, so I have a feeling that the overall crowd level in both parks will increase somewhat - even during the usual slower periods of time, but definitely in the second half of next year. Any time something new opens, people flock to it!
Of course, none of this means that the crowds will be unbearable. It will probably be more crowded than it would normally be for a September, but not insane. With the right planning (which everyone here on the DIS will help you with!) and preparation, you'll be able to get everything done that you want to do!
