SAD stands for "Serial Anonymous Downvoter".
It has been noticed by many JLU users that there has been frequent downvoting activity that indicates the presence of someone who seems to downvote questions for reasons that are not to do with helping to improve questions or support the quality of the site overall.
The defining characteristics of the SAD are:
Never offering an explanation of why they downvoted.
Downvoting questions very early, usually before anyone else has
voted, arguably so fast it's debatable that they could have even really thought about the question.
Possibly downvoting certain people more than others.
Whether the SAD is a person with some kind of agenda or not is not clear, and probably can't be determined, at least not without some kind of administrative access.
Nonetheless, even if the SAD is apocryphal, referencing "the SAD" when an unexplained downvote happens conveys that persistent downvotes without helpful explanations about how questions or answers could be improved does nothing to support JLU or help anyone learn Japanese.