A possible interpretation (of any administration resignations in positions that require Senate confirmation) is that Trump wants someone in that particular office that would not have gotten through Senate confirmation before the white hats secured a true majority.
(Although Republicans held a Senate majority, enough Rs were compromised that it was not effectively so, from a white hat perspective.)
If this is correct, it's not to knock Mattis (or anyone else who might resign to make way for a new nominee who couldn't be confirmed before.) I'm sure Trump has chosen good people for his appointments. But sometimes there's a reason for wanting a specific person in a specific office at a specific time, and with the incoming Congress it's possible for Trump to set up the chessboard with more freedom.
(There's also the option of a non-confirmed temporary appointee, not forgetting that maneuver as a way to set up the board)
Also, I have the sense Trump puts particular people in particular places for a stage of events, so someone leaving could mean that the specific skillset or experience or expertise needed for a specific upcoming stage calls for a change in personnel.
And if Mattis does wind up on the tribunal - that would be good! But the timing feels odd for that, I'd expect a January resignation.