These people must be following some kind of trail. And this is an automatic vote, with a conditionally selected value. Maybe they react to one of the tags? This happens, for example, with the photofeed tag. But I'm not 100 percent sure. Are you following some kind of trail?
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How do you make a trail on steem?
https://steemauto.com/dash.php
Look at this site. Here you can select a trail to follow and vote automatically for the participants of your trail.
Interesting, but less exciting than the idea of secret voting ring.