I'm just tired of the signal pollution, and would like to be able to use karma to honestly appraise the worth of my articles and posts, without seeing 80% of my downvotes come in chunks that correspond precisely to how many posts I've made since the last massive downvote spree.
EDIT to add data points:
Spurious downvoting stopped soon after I named a particular individual (not ALL downvoting stopped, but the downvotes I got all seemed on-the-level.)
One block of potentially spurious downvoting occurred approximately one week ago, but then karma patterns returned to expected levels. I consider this block dubious, because it reasonably matches what I'd expect to see if someone noticed several of my posts together and disagreed with all of them, and did not match the usual pattern of starting with the earliest or latest post that I had made and downvoting everything (it downvoted all posts in a few threads, but not in other threads), so I'm just adding for completeness.
Spurious, indiscriminate downvoting started up again approximately half an hour ago on Sunday (12/1/2013), around noon MDT.
Edit: And now on Tuesday, 12/3/2013, at 10 AM, I'm watching my karma go down again... about 30 points so far.
Edit: And now on Saturday, 12/14/2013, at 2 PM, I'm watching my karma go down again... about 15 points so far, at a rate of about 1-2 points per second.
Sometimes there is a good reason for this effect (not sure how often it applies): when you first notice a user and look through the last couple of pages of their comments, it might turn out that you don't like most of what you see, and so a significant portion of the last dozen comments get downvoted. Such voting is not noise, it reflects the judgment of the content. The reason for high correlation in judgment is not indiscriminate action, but merely that it is the same person that is doing the evaluation of a batch of your work. (It is easy to imagine how this pattern would turn to abuse, but it's not automatically abuse. There is also selection effect.)
What's happening to ialdabaoth is more extreme: about 98%* of their comments are being downvoted, that's happening repeatedly, and it even happens to comments that the rest of LW unanimously likes. To me that looks like indiscriminate & abusive downvoting, even allowing for the correlation in an individual's judgements.
* Skimming the last year-ish of ialdabaoth's user overview, I count 196 downvoted posts & comments out of 200. The most recent exception is a comment they redacted before anyone voted on it; the other three exceptions are these.