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.
This doesn't mean necessarily that there is a single person doing it. There is a simpler explanation using the LW mechanics: You see a noteworthy post (for whatever reason) and you want to know more about it. Naturally you click on the author an see lots of his comments. If you are rigorous it is natural to vote them all. If you generally disagree with them it is likely to look like a block downvote - what it is but not on priciple ground but simply because the it is a on-person batch operation facilitated by the LW UI. This can also happen the other way around. block upvotes. I seem to have noticed that I get some of those every now and then (and not necessarily directly following a post). I did this kind of looking at peoples contributions too. And surely I did block upvotes. No block downvotes but I could have.