I'll guess that the light-seal between the curtains and the body is poor, so that when the camera is pointed toward something bright, such as the sky, a little light sneaks past the upper and lower edges of the curtain and onto the film. The mirror would block most of the light on the upper edge of the film, which would explain why one edge is fogged more. An easy test is to shoot some frames, aiming the camera at the sky between frames, and some more frames but capping the lens between exposures and/or keeping the aperture stopped down the whole time.
EDIT: Also, light can be entering the mirror-chamber through the eyepiece. If the camera is pointed down, the eyepiece is pointed toward the bright sky. So for the experiment above, I suggest covering the eyepiece for the second set of test-frames.
HTH,
Mark Overton