Here is the question.
I'm not complaining, I'm just curious why it happened.
If you click the word "edited", you get the question's revision history. From there, click "Suggested" to get the history of that particular suggested edit. It says:
proposed 11 hours ago by an anonymous user
and gives some other details of the process, including the edit statistics of said anonymous user.
I'm pretty sure it means that an unregistred user suggested the tag addition and someone (with sufficiently high reputation) approved the suggestion.