I have one of these too. Someone is slowly working back through my comments systematically downvoting them. Given the rate, I think they're actually doing it by hand, and must have a browser window they've kept open for months just for this task. It's like they're trolling themselves for me, without me having to actually lift a finger. Some LW karma is cheap for such entertainment.
It was/is the same for me and others, too - small blocks of downvotes on old comments until they reach your first one, and then periodic block downvotes on your recent comments.
I also suspected that it is done by hand at first, but now I am leaning towards it being done with a bot/script (something adapted from reddit most likely), since it happens to many users and the pattern is quite regular over a long time.
Below is a message I just got from jackk. Some specifics have been redacted 1) so that we can discuss general policy rather than the details of this specific case 2) because presumption of innocence, just in case there happens to be an innocuous explanation to this.
So... thoughts? I have mod powers, but when I was granted them I was basically just told to use them to fight spam; there was never any discussion of any other policy, and I don't feel like I have the authority to decide on the suitable course of action without consulting the rest of the community.