Sadly, at some point in time in the past, someone logged into your Bungie.net account using one of the accounts you already have linked, went to the linking page in your profile settings, and added that account as a link. This was done manually by someone using one of your already-linked accounts. The only way it can be removed is to be able to verify the account for removal (this is to keep people from stealing others' Bungie.net profiles; it's a safety feature). If you don't have the login information for that account to be able to unlink it, then that account will not be unlinked. You can contact Sony concerning a possible hacking attempt and give them the information on the relevant account, but Sony is unlikely to give you the login information for that account. I would suggest checking any of your old emails and speaking with your friends, as it's possible it may be an old account you or a friend used to use that you just don't remember. If you ever 'bought emblems to use' from a person to use on your account, that could also be the unknown account that you allowed to be added to your Bungie.net profile.