Hope our experience with this situation helps...
If the Request Notification Rule 'Acknowledge E-mail Cc users when a request is resolved' is enabled and you set Status 'Resolved' it sends the notification. In some cases, the Requester email address or the addresses identified in 'Email IDs to notify' field was incorrect or no longer existed, in which case the rejection message from the mail server ended up in 'Requester' conversations, and as we have a new conversation set to re-open the request, it would do just that.
You probably already checked this, but just in case...