<p>I've encountered this very thing more than once in the past, looks to me like your film holder has a light leak. If you've loaded your film bottom edge first, the issue most likely, is where dark slide slot is. I've seen this mostly in older or aged, film holders that I acquired over the years. Due to lack of attention on my part, I ran through a number of sheets of film before the "light went on". Try loading & developing a sheet of B/W in the suspect slot without removing the slide, if the neg developes clean, you're good. Next, again, load a sheet, insert the holder into the camera, pull the slide and the reinsert, without firing the shutter, and develop that sheet, if it looks like your example that you've shown here, you've most likely found the problem. As mentioned above, there's a possibility that it could be a bellows thing, but the pattern is sometimes a little different, it tends to show across the entire sheet in most instances.</p>