OK, looks like I need to give a bit more information....

The film is 220. It was processed in a small tank, just the one roll by itself. It was loaded onto a plastic reel (the kind with the ball bearings) and I didn't notice any catches in the film as it wound. Seemed fine. I'm 100% certain there was enough developer in the tank, because I measure very carefully and know exactly how much fits into it. The way the film loads onto the reel, this frame would've been the outermost image on the outside of the reel. There are maybe five or six other frames affected, but not nearly to the extent of this one. I can see six definite bands on this negative; on the others, I can see maybe two or three, and they're much wider and more faint. Frank, I can't see any banding between the frames, but I'm not sure I would -- they're harder to see in the dark areas, and on some of the negs, I didn't see the bands on visual inspection, only after I printed. The affected frames are not all in a row. A few are at the end, and a few more are in the middle. I don't see any fogging on the film edges.

I initially thought something had dripped down the neg strip as they dried, but (duh!) that would've caused banding vertically, not horizontally.

Incidentally, I keep my house at 70 degrees, which is the same temperature as my developer, so I don't think temperature drop-off would be the culprit.....