Regardless of how many people are in the parks, Disney still considers October to be part of the off season. Halloween Time is popular, but it's not as popular as the holiday season. The holiday season is peak season. Summer is peak season.
However, October has gotten increasingly busy and more crowded over the years. There is no getting around that fact.
This year you have to take into account the Diamond Anniversary festivities. Disney has cut its Halloween Time season down to a bare minimum, but it still advertising it as Halloween Time while keeping the focus on the Diamond Anniversary. Many people are flocking to the parks to see the Diamond Anniversary things, as they probably think the nighttime shows will end after this year (and they probably will not end after this year). The 60th Anniversary is pulling in a lot of people, so October will probably be even busier than usual. Many people will flock to DL to see whatever there is to see of Halloween Time.
Within any busy period of time you will likely find days that are less crowded or more manageable, but they are not always easy to predict. If you go in expecting that you will find extremely low crowds, you will be disappointed. Everyone tries to do the same thing -- avoid the times that are known to be extra busy, and they end up making the less busy times even more crowded!
A number of things can affect crowds (and hotel bookings), including weather and special events or festive seasons. There could even be events that we don't know about which are happening in Anaheim or nearby, and that could contribute to hotels being booked. Prior to last year we didn't know about RaverDay in December, for example, even though it has apparently been happening for years. We found out about RaverDay when a CM told another member of this forum that the hotels were booked on a certain December weekend, in part due to the RaverDay attendees (also in part due to other things too, but that was just one of the reasons).