In short, yes, there is an automated system that attempts to detect "suspicious voting patterns", that may be fraudulent. See this meta.SE page for the details, and what steps you can take if you believe there are suspicious votes that the system has not detected.
That said, the behaviour you observed may not necessarily be fraudulent. Different users have different standards regarding the posts that deserve a vote. This is fine, as we do not expect every user to vote with perfect judgment, we only hope that the sum of votes will select the most useful answer(s) in aggregate.
This may lead to what you have observed. Note that this system both detects users that exclusively vote on a single user, as well as users that receive votes from a single user, so it is unlikely these votes are coming from a single user (unless the user would employ multiple sock-puppets. But why go that far and only vote a single time?).
Nevertheless, even if it turns out this behaviour is fraudulent, I wouldn't worry too much about it. A single fraudulent vote does not give a large amount of reputation, nor does it skew the score of answers/questions too much if people vote enough. (You may retort that people don't vote enough on CS.SE. I agree. However, my answer to that is that people should vote more. That includes you, dear reader) All this may be terribly unfair, but it barely interferes with the goals the score/reputation/voting system tries to achieve. So, please do report suspicious behaviour if you observe it (along the proper channels mentioned here, of course), but try to not feel too bad about the injustice that may go unpunished.
