This happened to me too. I think it's just automated script. See the MSO question, What is serial voting and how does it affect me? Here's how I identified it: I saw nine "undownvotes" logged to my reputation tab all at once. It does not say
serial downvoting reversed on my profile, but it does on the profile of the posts' author (twice—once for each of us, I suppose).
To be fair, before I figured all this out, my first instinct was a little indignation, an inclination to ask what was going on, and to defend my votes. That's probably only natural. Having looked into it a bit though, I can see that this script is somewhat clumsy, and that's not likely to change anytime soon (see also 1. The vote fraud script breaks legitimate polling questions - A serial downvoter's lament, 2. Should this be picked up by the vote fraud detecting algorithm?). I've repeated a few of my downvotes that got reversed, and intend to post the rest of my downvotes again over time, but I'm afraid it will be necessary to work around the system a bit to do so. This may require spacing votes out over time, and avoiding finding the questions you want to downvote through the user's profile.
The upside of this is that it will give us a little time to reflect on our votes before making them stick. In my case, I felt I already did this pretty well—I had specific reasons for each downvote based on the content of each answer, and I expect to see those same problems when I return to place my downvotes again. Nonetheless, I'll be glad of the extra objectivity I gain in the meantime, as this has been very hard to maintain (not just for me, though I think our upvotes reflect that @JoshGitlin was maintaining objectivity quite well despite the challenge). I haven't fully processed the implications of my apparently serial votes for the way I vote in general, and I'm not entirely sure I would want to vote in the same way on every other answer that shows the same kinds of problems but comes from a different user. It will be good to have some time to think about this, and the site will survive in the meantime.
An interesting question for meta SO that I don't dare ask over there might be, "Aren't there legitimate reasons for serial voting certain users' activities sometimes?" Yes, it seems like a big risk of bad behavior due to personal bias—we may have seen some of this here prior to the use of the "penalty box"—but I don't think it's impossible to remain usefully objective. For instance, as far as I can tell, this script would also stop us from serially upvoting each other's answers after following the links one-by-one on each other's profiles. I might actually like to do so! As @ChuckSherrington guessed, I have appreciated (and upvoted) most of your contributions that I've come across so far. You seem to have a lot of useful things to say, and that's basically all I need to see in a given answer before upvoting. As a professional psychologist, having recognized your talents, I should want to go learn as much of that useful content of yours as I can. (Despite appearances...) I've got other things to do with my life too, so I should do this as efficiently as I can by following your profile's links. I should upvote each of your answers that I do find useful in the end—that's a fundamental premise of SE's system, and part of what makes it work so well. It should only take me a minute or two to judge each of your answers as useful, because efficient skimming is an important skill in my profession, and you're generally not as "long-winded" as me (quoted that from an unconstructive comment I flagged just yesterday). So why should this be automatically reversed? For lack of a better system...?
Frankly, I suspect that's the case. No social system is perfect, largely for lack of better ideas. That's fine with me. After all, I haven't chosen to criticize the system here as it applies to serial downvoting, and I'm not sure I would. I'm content to just work around it, personally. I would appreciate feedback about this (from the mods, not from those whose posts I've downvoted) though, especially if this is something I shouldn't be doing. Again, I feel quite capable of defending my downvotes on objective bases, but I'd much rather just back off if I'm not actually helping the site like I think I am.