I have a post on flamenco with some photos, and one of these photos has developed like a life of its own. It gets many more hits than the post. It is only linked to itself, not to my post.
How did it become independent?
The first link is an “attachment page”. You probably had set the images to link to attachment pages at first (otherwise there wouldn’t be any comments on that att. page), then changed them and made them link to the real-size original instead.