There are a number of reasons for this, one of these being Nintendo’s past broadcast history.
This month an Indie World presentation was held, and typically when one of these is streamed a Direct happens a month afterwards.
So, looking at past scheduling the broadcast of an Indie World episode in December could indicate a Direct is coming in January.
The second clue which could prove a Direct is coming in January is Nintendo are running out of time to reveal the fifth Smash Bros DLC character.
The fifth and final DLC character for the first Smash Bros Ultimate Fighters Pass is due to be released in February.
Nintendo typically reveals these fighters during a Direct, bar the Joker announcement that took place at TGA 2018.