Does the effect go beyond the image boundaries? If it is only in the image, then it is in camera, leak from the front. If it extends to the edge of the roll then it is either out of the camera or a leak from the back.
I think Roland is trying to gauge if the leak is from the top or the bottom of the camera with his question?
So this has happened for a few rolls? On the rolls that it does happen, is it related to how you hold the camera? As in, only vertical shots? Or is is more evenly spaced, say, every second frame, or a regular pattern? This type of pattern to me looks like the film has been exposed to light from one end while tightly wound. My daughter once opened a bulk roll of mine that came in the mail and exposed the top, the pattern looked like this where the light had got into the sprocket holes and exposed the surrounding area. It could happen to individual cassettes too I'd imagine and give a similar look.
On the other hand, it looks like contrast is increased around the area, so it may be development...
Edit: the back door on my M6 is like this, it can be pushed in a bit on the top. Never had a leak from it.