Some users Notification settings being reset to all OFF...?
Using Jamroom
Kickbox doesn't have to move a profile to a quota - it can just "unsubscribe" the user.
We had another site report this with a few of their users, and there really is only 2 places in the code where the key can be changed - Michael outlined those above.
Michael's idea of it possibly being email antivirus that is somehow "following" or "loading" the unsubscribe URL (looking for malicious content) is about the only thing we think it could be (outside of Kickbox), so I think a solution for that would be to update the "unsubscribe" link so it has a button on the target page that is pushed to actually unsubscribe - that would eliminate that from being a possibility.