There is a lot of retaliatory butthurt behaviour on the internet.
You make one comment someone doesn't like and suddenly it's open season on everything you've ever said, regardless of it's worth. Somewhere like Reddit, someone will go through and downvote your last 40+ comments just because you got the better of them in a debate. Downvoting without commenting is the last vestige of the defeated. They know their argument can't hold water, so rather than concede the point, or move on, they go through and downvote anyone who spoke against them. While some comments are generally stupid enough that they need no reply (or further replies than the ones they've already received), someone who just abandons a discussion in favor of downvoting damages a community.
I can remember one exchange over on reddit, something on Korean language, where a native Korean chimes in as a reply to my comment "This guy is totally 100% right why is this being downvoted?" And it was all because of some other topic where a handful of butthurt children couldn't handle being proven wrong on a point so decided to run around downvoting anything else I'd posted within the last few days.
I've had it happen on Slashdot as well. Not in awhile, because I don't comment here as much as I used to (I used to frequently get mod points, but not that much recently). A few times, almost always after a debate with someone, the other party (I can only assume) would get mod points, and then past posts of mine, like ones over a week old, would suddenly all be moderated down as troll or something like that. I think I even made a post a few years ago about vindictive moderation.