Reddit also had it. I don't frequent other forums that use voting, but a forum I used to be part of had a user that would delve into the history of people he disagreed with and report year-old comments to get those people banned. Given that it's an easy way to hinder "opponents" I very much doubt it's LW exclusive. [...] Apart from willingness to use tools others would think immoral, no. I also don't think we need to go that far as an explanation. You only need one person doing it in a community as small as this one for it to become noticeable.

I think it's not so much a LW thing as a "one particular person on LW" thing, in which case there needn't be any particular connection with rationality or rationalism or anything else about LW apart from whatever circumstances led to us acquiring that one particular person. I expect (but have no evidence) that larger fora have occasional people doing pretty much any weird thing you might care to speculate about, including retributive downvoting. Maybe they get squashed by moderators or something. Maybe it's usually a small enough fraction of voting activity that no one cares.