David, no, it wasn't resting on its side at all. Thinking through it, If it had been on its side, considering the way the negs load onto the reels, the banding would've been vertical, rather than horizontal. I'm really at a loss for this one.

Could it be a problem with agitation? I do the standard Ilford-prescribed ten-second agitation per minute. Usually I invert four times during those ten seconds. Never had a problem before, but could that have anything to do with it?