I've also noticed that there seems to be a mob downvoting mentality. I seldom downvote already downvoted comments unless they are especially egregious, but recently it seems that you either get no downvotes or a torrent of them. Either that or people are playing sockpuppets.
[Edit: I think one way to mitigate this might be to make a user's votes viewable.]
[Edit: I think one way to mitigate this might be to make a user's votes viewable.]