This may be the reason: NAMM, the International Music Products Association trade show, January 23-26. At the Anaheim Convention Center (nearby).
NAMM is a huge reason. It is a gigantic international annual convention that draws thousands of attendees for several days. This fills all the local hotels, causes gridlock on the streets, and yes, impacts the crowds at Disney. These guests come from around the world and are staying steps away from
Disneyland so many plan to go to Disney while here. The Convention Center is literally across the street from DCA. It is a very big deal when Disney Management posts notices to Cast Member to avoid driving to work during NAMM and to allow plenty of extra time to get to work on time that weekend. If if don't like crowds and traffic I recommend avoiding Anaheim that weekend.