This has the classic look of an incorrect amount of either developer or fixer in the tank. That problem usually, however, happens more toward one edge of the film or the other from improper levels.

Another 'usual' cause for something similar to this is the film is not flat or it wrinkled on the spool and may be rubbing on the spool and on itself all the way around in the tank and creating a pocket for the chemistry to 'settle' in or be blocked depending on whether the it bulges or sags. This looks like the emulsion side was holding chemicals longer in one trough.

Self loading tanks (the plastic/resing twisters) as opposed to stainless are I believe more prone to this.

Just one roll? If so I'd guess a loading problem.