It's at least unusual in the sense that the top downvoter to a particular member has generally posted about three times as many downvotes as the second most prolific downvoter-- and the second most prolific downvoter is also a problematic downvoter.
How about the top downvoter for a different member?
It takes me weeks to get answers from support. I assume they're busy, so I'm not going to ask them extra questions, even though that's an interesting one.
It's possible that the stats show a different pattern if they're from people who aren't complaining.
I've gotten sufficient evidence from support that voiceofra has been doing retributive downvoting. I've banned them without prior notice because I'm not giving them more chances to downvote.
I'm thinking of something like not letting anyone give more than 5 downvotes/week for content which is more than a month old. The numbers and the time period are tentative-- this isn't my ideal rule. This is probably technically possible. However, my impression is that highly specific rules like that are an invitation to gaming the rules.
I would rather just make spiteful down-voting impossible (or maybe make it expensive) rather than trying to find out who's doing it. Admittedly, putting up barriers to downvoting for past comments doesn't solve the problem of people who down-vote everything, but at least people who downvote current material are easier to notice.
Any thoughts about technical solutions to excessive down-voting of past material?