I would guess that it is more to do with your wording. When i first read it I heard "dude there is already a post on this subject so why did you post another one?" Granted thats not what you actually said or probably what you meant. I think it stems from your use of the FYI, that can have a negative connotation. So 'FYI' and 'there's another thread already discussing' give your post a negative feel when read as if you are pointing out to the OP that they screwed up and there was already a post about this.
English + Human Beings + Intonation Free Text Based Communication = Miscommunication.