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.
With your and David's karma, it seems like you must have a fair number of comments. The 4xkarma limitation on downvotes suggests that it's someone who's got a fair amount of karma (or several accounts with a fair amount of karma if you're getting multiple downvotes per comment) doing the mass downvoting. That's just weird. It's hard to imagine which high karma person on LW would engage in individual persecution like that.
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.