MDL-32125, when a teacher or admin changes the subscription mode of a forum to automatic, all users enrolled in the course are subscribed, including new and pre-existing users.
If this is done accidentally or experimentally, there is no way to preserve the list of people who were previously subscribed to the forum and there is no way to undo the change.
There should be a warning when someone changes the subscription mode, either through the links in navigation or through the forum's instance settings, that the change will cause all users to be subscribed, perhaps asking for confirmation of this action.
As a rather awkward example of this, I changed the subscription mode of a forum on moodle.org by clicking the link for Automatic subscription then immediately changing this back to Optional subscription. Unfortunately several thousand users were then subscribed and wondering why they were receiving emails for a forum they never subscribed to.