Due to the geometry of the leak, which is so precisely defined, it doesn't seem that it would be from a simple gap around the camera back. I think the leak may be always present when the camera is oriented a certain way, but strikes the film only at the moment of exposure.
As Kino wrote, check the shutter.
If possible, hold the shutter open at B or T and with the back open move a strong flashlight around the camera as you look from the back. Try different speeds, as Kino suggested.