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.
It's happened to me as well. I argued in favor of an unpopular view and some joker down-voted all of my posts, even ones that had nothing to do with the view in question.
My solution is not to worry so much about karma. Even without the problem of block-downvoting, there are too many other problems with it to make it useful feedback.
Perhaps the block-downvoting problem could be handled by publicizing some of the information about peoples' up and downvotes. On a slightly different note, I would not be surprised at all if it turns out that some posters are operating smite-puppets to downvote their perceived enemies and sock-puppets to upvote their own posts. Or if there are pairs or groups of upvote-allies.
What was the unpopular view?
(I ask because there are some grounds for suspecting that certain particular kinds of view are particularly likely to be met with mass-downvoting; see elsewhere in the discussion here.)