I think David's more likely to be right than John... I also thought at first that the film had slipped, but if so there would be no image where the "perforations" are. However, there is an image, overlaid with a "redscale" type image of the perforations, so I too think that light is coming in at the back of the film.
Light leak effects tend to vary with the light conditions, and with the length of time the film stays on one frame (as opposed to several shots in quick succession).
OTOH I may well be wrong, too!