Yeah, like everyone else, itís hard to see it being chemical. Have you checked for light leaks in your darkroom?
I once had an A12 film back for a blad that got a little bent (I lent it to a mate**). It had a very slight light leak, which did not noticeably affect the film when shooting and winding on relatively quickly, and so, most frames were fine. However, some exposed frames left stationary near the leak for a period of time, say between locations at a wedding, etc, sometimes had slight to moderate fogging, in bands across the film.
Like I said, itís hard to tell, but leave no stone unturned, lest it may continue.