I have also seen this pattern, particularly from a specific user.
Example: data insertion using PDO
User @YourCommonSense answered the original question with a flippant rant:
I don't know what's particular problem with your insert, but your implementation is just terrible.
He goes on with a long reply about exactly what's wrong with the OP's code, but without addressing the question.
Two other users, myself and @moskito-x, answer the OP's question and give helpful and correct answers. Despite what @MartijnPieters suggests, this is not a case where the abusive user answers more correctly.
@YourCommonSense marks each of our answers down, and writes a comment meant to justify the downvote (he has since deleted his comment on my answer, but it was dismissive about my answer as merely correcting the OP's typos).
I understand we cannot know for certain that he gave the downvote just because he made a comment, but the fact that they happen so close together in time, and that I have seen the same pattern on several other questions, makes me think he is in fact the downvoter.
At least his negative comment to @moskito-x was completely wrong ("Insert cannot fail because of incorrect data.").
This isn't the only instance. @YourCommonSense is an example of user who is rude, disrespectful, and abuses the downvote system. He routinely downvotes other correct answers, and justifies them with trivial or outrightly false objections. He routinely posts grouchy rants about other people's code style, without answering their question.
My point, though, is that the StackOverflow site does not give us any means to flag such users who repeatedly behave badly. We can flag their individual offenses, but there's no way to raise the broader issue to the moderators.
What I'd like to see is a way we can call for an intervention. I'd like moderators to confront the user and tell him, "behave more respectfully, stop downvoting other correct answers, and don't post without addressing the question -- or you'll be banned."