This looks to me to be that your processing chemicals didn't adequately reach the center of the frame. If you somehow managed to too loosely fit the film on the reel or otherwise make the film nearly touch in one area, this might result. You might be able to re-fix and re-wash the film and remove most of this fog. Your agitation scheme seems adequate for developing, did you give the same agitation while fixing or did it sit longer with less agitation?

Peter Gomena