It's happened to me too. I used to get so happy when I first started out then I learned that 90% of them are just bots. There is a steemian that once told me that some bots just be waiting for somebody to use a tag for the first time and then they follow him. To what end I don't know. I was was able to confirm this last week when I posted with dlive and got at 30 followers in less than 5 minutes.
Still even when I'm not using a new tag or even posting I do sometimes see them following then unfollowing me. Smh
Who knows, probably. I pretty much use the same tags for all of my posts because I mainly blog about food and they still follow every time, I haven't seen it happen before, it's just the last couple weeks that I started noticing them follow like crazy haha.