Ah, okay, tags.pub.
It's a service that allows folk to follow a hashtag by following an account that is named the same. (Not all fediverse software necessarily provides for hashtag following. Even mastodon borks this functionality by limiting how many one can readily follow.)
They harvest tags from relays, these being servers that help less prominent nodes in the fediverse get seen.
So, all it would take would be someone who boosts your posts (even if they don't follow you) being on a server that participates in a relay that tags.pub subscribes to. All hashtags it sees automatically become automated accounts.
They say you can opt out by including a hashtag in your profile. Is probably simpler to just block their whole server.