It's certainly not insufficient developer; as has already been correctly stated, that would give clear, not black, results.(posted 8381 days ago)It might be insufficient fixer, though unfixed film will start out looking opaque milky white, and only turn dark after extended exposure to light (or fairly brief exposure to developer).
Light leaks will rarely produce a sharp edge, and will usually be very uneven. I'm guessing it's a light leak somewhere. It might be in the camera, or in the film cassette, or somewhere in the development process. Hmm, was the film ever subject to X-rays at an airport?
Unless your motor winder requires that you use a different back or do something else that affects the light seals, I doubt the winder is causing this.