Hi, recently that seems connected to “addressive” spam checkers and email scanners. If look at the details of that subscriber, on its log panel you can find the IP address from which the unsubscription has been started. Could you report it so we can check on Maxmind?
An help to avoid this problem is to enable the double steps unsubscription, those spam scanners usually do not go so deep in surfing the blog links.
We’re studying a solution to block those scanners but they simulate perfectly a regular browser, of course they don’t want to be blocked.