I have tested this hypotheses the first 3 times it happened. If you check my comments, you will note that most of them are not the sorts of things that would be downvoted normally - they're either clarifying questions, or things that multiple people had previously up-voted.
If 6 posts with high positive karma all get downvoted at once, along with every other post made within the past few days, I'm inclined to believe that it's retaliatory, especially if it always happens within a few minutes of a particular poster making a post, and especially if that particular poster is someone that I once had a contentious argument with about a political subject.
For what it's worth, I've just looked over a few pages of ialdabaoth's comments and they seem consistent with this hypothesis (although the bulk of the comments are now at 0/50%, suggesting that someone subsequently upvoted them back to zero). And ialdabaoth has a ~70% positive rating at the moment, which seems anomalous given the content of their contributions.
As for what to do about it, I'm not sure either. That said, I don't want to see it become a common practice, as it corrupts the information content of karma totals.
So whoever is doing this, please cut it out. You're soiling the commons.
People who go back and downvote every post or comment a Less Wrong user has ever made, please, stop doing that. It's a clever way to pull information cascades in your direction but it is clearly an abuse of the content filtering system. It's also highly dishonorable. If you truly must use such tactics then downvoting a few of your enemy's top level posts is much less evil; your enemy loses the karma and takes the hint without your severely biasing the public perception of Less Wrong's discourse.
(I just lost over 200 karma in a few minutes and that'll probably continue for awhile. This happens to me every few weeks. Edit: I mean it's been happening every few weeks for a few months for a total of only three or four. Between 400 and 700 karma lost total I think? I don't mean to overstate the problem.)