Leads can be auto unsubscribed due to default Feedback Loop setup with the ISPs listed on this page.
You can use the following filters to find leads that have clicked the SPAM button in your emails:
Filter 1: Data Value Changed
New Value: True
Reason: Contains, Customer Complaint Received from ISP.
(Optional to Specify what Email Domain)
Filter 2: Email Address
Email Address: Contains, @domain.