He did switch to the job of “NCIS Historian” back when he passed the Chief Medical Examiner job on to Palmer back in Season 16.
Has he already been out of the action long enough to be shocked by the state of some of the NCIS crime scenes?
Okay, maybe not so much about the cake and streamers.
Ducky actually looks like he’d rather be in a quiet library somewhere, or perhaps a morgue, as opposed to any location where he’s having to lock eyes on a freshly discovered corpse.