I discovered a source of the "accidental unsubscribes" recently.

Lindsey is subscribed to my newsletter and loves it. She forwards one of my newsletter emails to Susan, who likes it but does not love it and hits the 'unsubscribe' button at the bottom. That action by Susan, seems to unsubscribe Lindsey.

This leaves two people unhappy. Me for having lost a fan, and Lidsey for having lost the next edition of my newsletter from getting in her inbox.