I am gradually updating in favour of the hypothesis that at least one of the admins either approves of mass-downvoting as a means of influencing LW culture, or else has a strong enough dislike for the sort of ideas that appear to be be the targets of mass-downvoting at present that s/he considers the mass-downvoting to be a good thing.
I have a prior that admins don't consider karma important and think of up/downvoting issues as middle-school-level status/power games. "Mommy, he hid all my pencils and wrote a bad word on my locker door!"
I don't think so: measures such as the hiding of below-threshold threads (pushed for by EY) make karma less unimportant that it used to be.
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.