The site automatically adds tagme when a post otherwise has zero tags, either because it was uploaded that way or because a vandal tried to blank the tags.
We could add a minimum tag requirement but the question is what should it be? Given difficult-to-tag posts like the one kuudere points out the highest it could probably be is 3. There are no active tagcount:<=3 posts. But there are also people who upload first and tag later, so a tag minimum may impact them.