Also possible that it is a light leak through the light trap on the cassette if you wind the film right in and expose to a bright light source. This will always happen with IR film if you take the cassette out in the daylight, and the banding looks just the same...I know, I've done it!
Realize that this is not IR, but there is a slight chance this could happen?
Yes, leaky cassette makes sense since its only a couple of frames.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh