This is due to the film being only half covered with developer and fixer. What you're seeing as "fogged" is the side that was toward the top of the tank, and far from fog, it's undeveloped halide.

There are two reasons this might happen.

One is if you're only using half enough solution (for a single roll) or not quite enough (for two or three rolls in a single tank). Most stainless tanks want 8 ounces or 250 ml for a single roll of 35 mm, and multiples of that for larger roll counts, while Paterson (one of the commonest plastic tank and reel systems) want 290 ml for each roll of 35 mm (I'm not as familiar with Jobo and there are a couple sizes of those, but they're marked with the volume needed).

The other reason is if, in a Paterson or similar tank, the reel isn't seated all the way down on the column, so it rests in the bottom of the tank, so the correct amount of liquid doesn't cover the film. This might also happen if the column and reel assembly is installed upside down in the tank, placing the reel at the top instead of the bottom. This generally won't happen with stainless tank systems, as there is nothing (usually) to hold the reel up inside a tank that would hold two reels.