If you did not click on the Unsubscribe link yourself, below are 2 common scenarios where this could have happened.
You have forwarded the New Lead email to others. Someone on the receiving end could have clicked on the Unsubscribe link.
You replied back to the email, but replaced the default reply to email address with the lead's actual email. The lead could have clicked on the Unsubscribe link.
To restore the ability to receive leads, please see the below article.
Additional questions, please reach out to email@example.com