On Mastodon, there’s a feature that automatically suggests accounts to follow when people first join a server, and when they click on the For You tab in Explore or Search. It is based on how many people on that server follow the account and boost its posts, and server admins can optionally add suggestions manually too.
If you don’t want your account suggested to others:
- log in through your server’s website
- Go to Edit Profile > Suggest account to others
- Make sure the box is unticked and click Save changes. (If you want your account suggested, tick the box and save instead.)