My best guess is some sort of account or system restore, but I'm not sure if that's even possible within their architecture.
Wild idea is that they restored/activated all of their old support emails, but then all users would be seeing this who have contacted them, and if true how would emails behind the mailbox password be recoverable? Maybe if every email is stored individually, tagged to your account, and appears if your mailbox password is a match for the decrypt.
You'll have to ask
@ProtonMail to comment to get a more certain answer.