I would guess that this bimodality correlates with one in the personalities of those who become moderators - on the one hand, those who are willing to put a lot

I would guess that this bimodality correlates with one in the personalities of those who become moderators - on the one hand, those who are willing to put a lot of effort into keeping the community relevant, and on the other, those who abhor any perceived irregularity. On occasion, when I get invitations to participate in moderator elections, I look to see if I recognize any of the candidates. Once or twice, I have looked through the candidates' resumes, and one thing that struck me was that some of them did a lot more down-voting than up-voting. Make of that what you will.