You are not using 220 film with the camera on the 120 setting by any chance? The 120 setting assumes film plus backing for the pressure plate position.

The borders showing the effect are adjacent to the film rails. The length of the effect matches the length of the frame, has a sharp boundary. I do not see any signs of a double image. If the film had moved due to the back not being fully latched (you have to press both corners of the back on a C330 to ensure full and even latching) it would have to be during exposure. And that should cause a double image.

Low level light leak seems to be a possibility. That would imply the long edges of the back, or possibly the internal lens change baffle if the lens was removed. The shape of the fogged area suggests a uniform effect. But since the effect does not involve the edge of the film, it is unlikely to have originated anywhere except the film gate.

If it is camera body fogging you should be able to replicate it by doing everything you would do normally, but expose the frame with a lens cap on.

The pictures here show the film gate for a C330, which may help discussion.