It takes some amount of people flagging it before it shows up as [flagged] but flagging submissions affect rankings before it reaches that point. On top of that

It takes some amount of people flagging it before it shows up as [flagged] but flagging submissions affect rankings before it reaches that point. On top of that "Ask HN" posts also have some modifier on them that makes them drop quicker, these two effects seem to be cumulative, explaining why it dropped so quickly