If the email has a link to the unsubscribe page hard-coded in the email (or template) HTML, Marketo will see it as a simple link, like any other link, and will include it in reporting as a Click Link activity.
Marketo will only automatically omit clicks on unsubscribe links when either the {{system.unsubscribeLink}} token or the default email footer is used in an email. These links will will show up as Unsubscribes instead of link clicks.