"Venting" is far too mild a word. It's an abuse of the voting system, and we all know it.

how do you know any given chunk of downvoting is indiscriminate?

When a user casts all, or nearly all, of their votes for the day, for days on end, merely downvoting old nodes of one particular user, that's not doing one's moderatorily duty, that's abusive. No one says: "Hm, I discovered this one post by Jo Blo which is pretty bad, I'll downvote it; now I'll spend a few hours checking to see if anything else he's posted also merits a downvote." No, rather, they say: "Jo Blo is a blight on the Monastery and my only power is to downvote all his nodes." And that's what we want to discourage.