This is how I understand this:
Schools have lots of applicants, with many being qualified but in a 'grey area'. Schools want to get the most out of their applicant pool so they review these applicants while waiting for other applications to see where the grey area candidates stand. Once the secondary deadline closes, schools review the remaining apps and can rank the applicants. I think that the last big of movement is a result of that.
I honestly think many of us will see more movement closer to the deadline or soon after. This includes rejections.
IMHO, once you get into feb (i.e. 1+month after secondary deadline) in places, the new II's that come out are due to people that are withdrawing acceptances and interviews and a school is trying to make its numbers. Those are the last minute hail mary things for the schools. Yes they still want you, but the circumstances are far different than they were in aug/sept. I think these are still very viable interviews, as people get in off these spots.