Check the lid and the cap of your processing tanks for cracks and light leaks, even a tiny amount would cause something like this. I had this happen years ago, a tiny tiny split on the plastic lid caused a light leak along the edge of the tank which went down the side, fogging just the last few frames, I was rolling from the front of the cassette. I figured it out by processing two rolls, one rolled from the front (as in pulling film out of the cassette directly onto reel) and one from the back (as in pulling all film out then starting the reel with the end of the roll and moving forward.) What tiped me off is that with two different cameras, loaded in a tank at such, the same areas in reel were fogged. Tiny crack, doesn't take much for fog. Also check to make sure your overhead floro-tubes are not glowing, keep them off as long as possible before rolling.