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.
That's a short enough change that it could be from someone simply reading an old thread. I've had occasions where my karma has gone up by 10 or 15 points with no changes to any of my recent comments. On the other hand, I have had at least two occasions where all my recent comments on a variety of subjects had their total reduced by 1. That's more targeted. I don't think people who are engaging in deliberate, targeted downvoting are generally going to bother to go back 15 pages of comments and then vote from there. On the other hand, the behavior is already sufficiently irrational, that my ability to model such an action is iffy.
Irrational behavior doesn't equal random behavior and is typically easier to model than actually rational behavior. Think of a drunk thug, only capable of following very few rules-of-thumb which do not even accomplish his actual aims ('irrational').
Compare HPMOR!Quirrell and a redneck neighbor.