Mine started happening about 3 weeks ago. I've always had my account set for no e-mail notification.
I went into my account a couple of days ago and noticed that it was switched to email notification for subscribed threads.
I switch it back, but still got e-mails. I even started getting e-mails for older threads that I knew had no notification setting.
This evening I haven't noticed any new e-mails though. If I get them again, I'll updated this thread.