It's extremely simple database queries, in plain english along the lines of "return list of downvotes by user A, and who was downvoted," "return downvotes on posts/comments by user B, and who cast the vote," and "return lists of downvotes by user A on user B."

And then what will you do with that data? If you find that GrumpyCat666 cast most of the downvotes, does that mean that GrumpyCat666 is a karmassassin, or that GrumpyCat666 is one of the gardeners?

(I can't find the link now, but early on there was a coded rule to prevent everyone from downvoting more than their total karma. This prevented a user whose name I don't recall, who had downvoted about some massive fraction of all the comments the site had received, from downvoting any more comments, but this was seen as not helpful for the site, since that person was making the junk less visible.)