I had the same problem discussed on a german discussion forum 1-2 weeks ago. Answers were not very helpful. It clearly araises from different developer agitation on the edges and center of the roll film strip. I never saw it on 35mm. It is highly dependent on the type of developer and film used. "Thick" emulsions and highly diluted developers show the strongest density geadients. e.g., APX400 in Rodinal gives unusable negatives when rotary processed; on the other hand, Tmax 100 in Xtol stock has almost (almost!) no visible densitiy differences.(posted 8231 days ago)You can mistake it for a light leak problem, but a light leak effects mainly the unexposed border (and give more base density), while in uneven processing the boders are clear.
I suppose, many photographers burn in the edges of their prints routinely, if they have the "feeling", that they are too bright, but don't think further of the origin of the problem. If you have seen the effect on some prints, you start to recognize it on many others (a learning effect, not a phobia).
You can minimize the problem with (*)prewash 5 min (*) maximum rotary speed (*) use highly active developers (Tmax, microphen, ultrafin sf....)
I dont want to mess around with my negs, so I quit rotary processing of B&W-rollfilm completely and changed back to inversion.
Georg