I am also confused by how light is getting to the part of the film which is behind the back's mask, in this case right to the very edge of the film. Interestingly the foggy area is shaped just like the image itself, with the little notches on the corners being extended as if light is being thrown threw the back mask at an extreme angle (which supports the flare theory I think?).

In racking my brain about this I do remember that at one point during the shoot (not sure if it was this pic) I did attempt to remove the back before putting the dark slide in - I was working very quickly using two backs (colour and b/w) for every shot and I moved the sliders (they only went part way because of the camera's failsafe) to try and unlock it and pulled a little on the back before realizing that it was not coming off - do you think this could have caused the problem?