As an answerer, I have the habit of voting up the questions I answer. It is because if they wouldn't be useful in my opinion, I wouldn't answer them.
From a rep collection perspective, I would say voting the question down, distracts the other answerers, but it also distracts the viewers. Thus, the chance that my answer will be voted up, significantly decreases.
Although the chance of - in the lack of other answers - finally my answer will be accepted, significantly increases. Particularly because it would be a "good policist, bad policist" game for the OP: he would see that his question was unfairly downvoted, but despite that, this nice guy answered it.
Beside the major ethical side of the trick, there is also a strategical problem: if my question gets accepted, it results only +15, but I can get this +15 only once per answer. While I can get an upvote any times. Thus, from a rep collecting perspective, I see more results in upvoting the question as voting it down.
The overwhelming majority of the reputation gained on the sites is coming from answer upvotes. It is far more as the rep gained by accepts, edits, bounties or questions. Likely all the experienced answerers know this very well. If we want to get many rep, the obvious strategy is to maximize the answer upvotes, every other has only secondary importance (including the possible downs).
However, I also think that on the SE network, many people likes to harm others on many ways, on perfectly irrational and unsaid reasons. I think, these unsaid reasons may be in many cases, that the destructive person thinks, he would gain something with the harm. Knowing their this habit, I think it is quite possible that it is more wide spread phenomenon as it would be reasonable.
However, I think the trick would work.
The statements of others, saying that this would attract likely a mod intervention, is not realistic in my opinion. A single, silently downvoted question, where the interaction between the downvoter and the answerer isn't trivial, probably wouldn't even attract the attention of the CMs.
And probably even the mods wouldn't see anything, because also they can't see induvidual voters. Their voting irregularity detection is tuned against the voting chains, not against such tricks.
Although it is possible to catch somebody having a habit to answer -1 voted questions.
If the answerer has 2000 rep, then he can relative silently edit the question, and revoke (or invert) his downvote on his alternate account.
Also this likely wouldn't excite the CM control, because there are no votes between different users of the same induvidual person.
Although I think, they could (and maybe they do) watch also this trick. Collecting the users with the habit of answering and downvoting questions, would be a singleline query with database access.
In my opinion, the main reason, why it is likely not a wide-spread "strategy", that the lost upvotes (by the decreased number of viewers) results more potential loss, as the increased probability of the accept.
In the case of the SE, I would more likely watch for an opposite pattern: the cases, as the seemingly "concurrent" answerers comment eachother's answer and then everybody silently votes everybody up.
I have seen a number of high K users do this.
Unless you're an employee of the site, you don't actually have the ability to see who votes for what, so no, you haven't seen that. You might suspect it, but that's all.