I'm leaning away from the light leak in the tent theory. The pattern seems too consistent for that. I would think that light coming through into the tent would affect the film differently depending on how long sections of the film were exposed. When I put film into the reel in my darkroom, I am not consistent in how fast I do it; some parts of the the film would almost certainly be exposed to that light longer than other parts.
I can't think of what this might be, but I don't think it would be related to light coming into the tent.