The unsubscribe link is a static link with a hidden private token.
When a subscriber (Alice) forwards a newsletter to their friends (Bob, Chuck), the token gets forwarded as well.
That means, that Bob and Chuck can unsubscribe Alice.
This might happen when Chuck wants to stop reading from us, thinking the unsubscribe button will work.
But in fact, Chuck is unsubscribing Alice, our actual subscriber.
End result: Alice wants to read from us but was unsubscribed and doesn't understand why she doesn't get our emails any more. (She didn't even notice the unsubscribe link…)
Solution: Send an unsubscription confirmation email.