Is this a random process and I've been unlucky or have I been singled out for some reason? Have I been a bad reviewer (gasp)?
The review honeypot posts are inserted automatically, and I'm fairly sure it's a random process.
We do not single out users and put them to the test. If we observe unusual reviewing behavior, we'd rather check that manually and contact the users in private about it. The system takes care of the rest, i.e. if you fail too many audits, you will be temporarily banned from reviewing.
Once every few dozen reviews I can understand, but the frequency with which I've been getting them seems much higher.
You only got four audits within the last 30 days, so, that's really not a lot. The frequency of tests should be relative to the number of reviews you make in total, which also makes sense since they are supposed to catch reviewers who are just there to aim for the badges and click through everything they see.
Just to illustrate that we're not singling out users and that the process tries to test users who participate a lot in the current queue: The suggested edits queue is always visible to moderators, while it only shows for others when there are at least 2 edits pending. This means that moderators get to review edits much more often than regular users, and so they also get more review audits.
If others share my feeling that these tests are coming too often, could we dial their frequency down a notch?
I can understand that if you're trying to be an honest reviewer, the feeling of having someone (or something, in that case) watching your actions seems annoying. I think most of the reviewers would say that they're of course trying to do a good job.
Please keep in mind though that – other than through these tests – we have no way of finding out whether someone's really trying to clean up the site, or is just here for the shiny badges. Unfortunately, for these users, tests sometimes don't come often enough. Once they're banned, they leave a trail of unactioned posts behind, many of which should have been reviewed more carefully.
All in all, I'd be not inclined to ask for the number of review tests to be lowered—quite the contrary. If you could see what kind of posts underwent review and simply were dismissed (including obvious spam), you'd be okay with getting tested here and then. I'm sure that these tests aren't the perfect solution to the underlying problem, but they are working pretty well at the moment…