Well, that solves one mystery. Upon uploading my last post I immediately received two likes and a follow from three blogs I had never heard of before. My guess is that people are running bots keyed to tags, and immediately following/liking posts based on those tags.
The goal is twofold: to stroke my ego and to get me to follow/like those blogs back. In this way they build up their audience base. This is how people do publicity in WordPress.
I am guilty of this as well, although I am less blatant. I populated my blogroll with active bloggers I read using other mechanisms, in the hope that they would notice and follow me back. So even though these referral spam tactics feel skeezy to me, my hands are dirty as well.
The lesson to be learned from this is that my tagging strategy is broken. I should be loading my posts with long, straightforward lists of tags to attract the audience I would like to have reading me. As a stubborn old git I am reluctant to do so, so I guess it is another life of obscurity for me.