Supposedly a trick people use on reddit is to downvote all of the other comments on an article when you're making a comment, to give your comment an extra bit of boost, especially early on when there's few upvotes. I would hope the reddit algorithm catches that, and with HN requiring a pretty high karma threshold for downvoting comments should keep people from doing that.

Isn't it possible that people view the karma threshold as a barrier to entry they can take advantage of? What's to stop some organization from making 100 accounts that accumulate karma and then start engaging in low-signal astroturfing/shilling that is harder to distinguish, especially if others cannot counter them without similarly organizing a group and accumulating karma over time?

It is a curious occurrence but that has happened more than once for me. It's also a good lesson in ettiquette and patience: editing your comment to challenge downvoters almost assures that it will not rise positively again.