See https://phabricator.wikimedia.org/T95135#1217734 : the user had just unsubscribed, but changing projects/summary via the edit window forced them to be subscribed again.
There was some report about comments automatically adding people to subscribers/cc, but with Phabricator's search I'm unable to find it...
Quoting or otherwise mentioning another user in a comment might be more common, but task editing is more important and users shouldn't be scared of improving tasks for the fear of stalking unwilling users.
T76993: Quoting shouldn't readd me to a task I've unsubscribed from
T109359: @mentions add subscribers even if they were explicitly unsubscribed already