There's a fairly common pattern where someone says something that a small handful of folks downvote, then other folks come along and upvote the comment back to zero because they don't feel it deserves to be negative, even though they would not have upvoted it otherwise. You've been posting a lot lately, so getting shifts of several dozen karma back and forth due to this kind of dynamic is not unheard of, though it's certainly extreme.

Concerted, not necessarily organized. It's possible for one person to put a pretty big dent in someone else's karma if they're tolerant of boredom and have a reasonable amount of karma of their own; you get four possible downvotes to each upvote of your own (upvotes aren't capped), which is only rate-limiting if you're new, downvoting everything you see, or heavily downvoted yourself.

This just happens to have been a sensitive issue recently, as the links in JoshuaZ's ancestor comment might imply.

Block, meaning organized effort?

I understand block downvoting as a user (one, but possibly more) just going through each and every post by a certain poster and downvoting each one without caring about what it says.

It is not an "organized effort" in the sense of a conspiracy.

Blockvoting may or may not be going on in this case, but at this point, I also assign a high probability that there are people who here downvote essentially all posts that potentially seem to be arguing for positions that are generally seen as to be on the left-end of the political spectrum. That seems include posts which are purely giving data and statistics.