Mailing list member received unexpected opt-out message
In civi, you have two ways to be removed from a mailing: unsubscribe from the group or opt-out (from any mailing).

The later doesn't remove the contact from any group, but change the status of the email and automatically exclude the contact from any mass mailing. 

When you do to the contact summary, do you have a red icon next to his/her email? 

If you want to have the contact receive mass mailing again, you need to edit the contact and uncheck the opt-out (beware, you should only do that for contacts that actually want to receive emails)

As for why she/he was opted out, it's likely the email was forwarded around, and one of the recipient that received the forwarded email clicked on unsubscribe. Obviously, instead of preventing friends to forward you emails, it did unsubscribe the original recipient, hence the opt-out message.