If it were any sort of light fogging then the pattern wouldn't 'go around' the sprocket holes - it would be even it that area and the sprocket holes would be just holes.
I still think it looks like something was sticking. Possibly Ilford film doesn't like freeze/thaw cycles - but if one 'wrap' of film stuck to the one below it then there should be artifacts from two sets of sprocket holes.
What were the common factors between the HP5 and FP4? You mentioned freeze/thaw cycles. I have done that countless times and never seen anything like it. I have a personal crusade to get the famed 'condensation' and all the sorts of other postulated evil doings due to not thawing film in the politically correct manner: I make it a point of honor to take film straight from the freezer to the camera to taking pictures whenever I can. I have never had any sort of problem with this practice. Thermodynamics says it is nigh impossible to get condensation on the film (engineering degree - EE, but one year of thermo). But then, I shoot Kodak film exclusively.
Was the pattern exactly the same on the two instances? It would be a rare thing to happen to one coating line, that it would happen to two - a case of Douglas Adams' "Infinite Improbability Principle"?
I love a good mystery...