Checking my rep this morning, I came across this:
I know that I haven't been doing any 'undownvoting' - in fact, I was in bed when this occured. Are there other things which can cause this to happen automatically?
You were caught by a script that triggered on you for having performed what it identified as fraudulent serial downvoting on a user. In your case, the script reversed all of your downvotes at the same timestamp that your undownvote entries are listing. You received 1 reputation back for each of the reversed downvotes that you had cast on answers.