Any alerts that are sent via automated email are required by law to include an unsubscribe link. This link executes the unsubscribe_from_alerts API and sets the Alert User record to inactive.
​
​

If this API is being triggered unexpectedly, check to see whether the alert user is forwarding alert emails to others. If another user clicks the unsubscribe link from the forwarded email, the original user will be unsubscribed.