Don't know if it's perfect but I think the phrasing you'd used downthread "Downvoting all of a user's nodes with prejudice" would read as clearer / less vague than just "Massive downvoting" (and then giving e.g. "Exclusively downvoting a given user across multiple days" as an explicit example of "with prejudice"). Then again I might could see just "Massive downvoting" so long as there were a couple of clear, tangible examples provided afterwards as to what's considered such an abuse.