I haven't seen this problem quite so much on Hacker News -- or at least, it's nothing compared to more mainstream sites. On Reddit you can actually watch (part

I haven't seen this problem quite so much on Hacker News -- or at least, it's nothing compared to more mainstream sites. On Reddit you can actually watch (particularly on slower sections, like proggit) as single people come into the thread and every single post expressing opinion X loses one point. I don't think it's caused by disagreement per se. I think it's because people (whether they realize it or not) want to promote their opinion on certain topics. An upvoted comment expressing a contrary opinion seems like an endorsement of that opinion by the community (even if it isn't). So they downvote it to try to reduce the appearance of that endorsement, thus promoting their own opinion in its stead.