This is a community-moderated platform. As such, people will do all sorts of things without explanation. It's the nature of the beast.
As for the specific instances you mention, I can't help but notice they're on questions with close votes. If a question is really very far below community standards, I've downvoted answerers just for answering. I don't believe it's good for the community to entertain and encourage noise. I don't see that issue with these particularly questions, but it is a possibility that somebody thought differently.
Lastly, the system has tools to detect and correct systematic voter fraud: What is serial voting and how does it affect me?