December monthly updates are only special because of MS company culture.
Monthly updates don't magically appear out of the air. They are planned months in advance, first tested in Insider builds, then promoted to Monthly Previews, before being officially released as the Monthly Update. Unless it's an emergency fix, most changes have been working their way through the release pipeline for some time.
Everyone in Redmond leaves for vacation during the US holiday season, from Thanksgiving/Black Friday until the New Year. Many important decision makers are absent, so work is planned around the holiday schedule. Less changes are added for the update scheduled for December, and now MS normally skips the December Preview.
Security updates are always included in Patch Tuesday, regardless of the month.
Which implies a lot of changes are added to October/November and January/February.