What is causing this?

To me, that looks like you didn't quite have enough developer in the tank to cover the roll properly. I often get that on the top roll when I do two 35mm rolls in a two-roll Paterson tank with 600ml of developer.
 
Yeah, I always use 600ml for a single roll of 120. I can't remember what the Paterson tank suggests but I think 500ml isn't enough.
 
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