You have found the phrase 'lead unsubscribed from list unsubscribe.' as the listed reason for a person being marked as Unsubscribed, or you are seeing it in the details of a 'Unsubscribe Email' action.
This message appears when someone has unsubscribed using a special 'list-unsubscribe' option automatically contained in Marketo emails, as well as most other reputable marketing emails. This 'list-unsubscribe' feature is an email address contained in the background header information of the email. Any time someone sends an email to this address, their email address is unsubscribed from your database. This is almost always done by that person's email client, using their inbuilt 'unsubscribe' features, such as the ones in Outlook, Gmail, or other common email clients.