This is a bug in the forum software, which really seems to be causing some concern! When a user deletes their account, existing posts from that account are labelled as posted by 'unknown'.
However, confusingly, replies to those posts appear as if replying to posts by the user name of the person that's reading them.
So you will see replies to those posts by the deleted user (in this case 'soon-cat', I believe, AKA 'eyebrowfetish', and several other user-names) as if they're replies to you. I'll see them as replies to me. And, I assume, everyone else is seeing them as replies to them!
It's very bizarre, but no cause for concern or worry. No-one other than you is seeing those replies as being made to posts 'from you' (that you didn't make)!