If the commercially done rolls from the same camera are ok, it kind of leads to your processing. Is the fogging always at the bottom or do you have rolls where it is on top?
There is one possibility regarding the camera I can think of. The right side strap lug came out of my M4-2, and I discovered that it's anchored into the film chamber. I don't know if the M6 has similar construction, but you might check to see of the strap lug is possibly loose.
Otherwise, what is your processing workflow, darkroom loading or changing bag, what kind of tanks and reels? If you're using plastic, have you checked for cracks? As a test, you could sacrifice a roll or part of one, and process it without exposing in the camera.