Is the emulsion perfectly smooth in those areas? I wonder whether this is some weird form of reticulation or the emulsion sloughing either during manufacture or processing. Just a guess.
If it is some sort of a light leak, it had to occur while the film was moving.
I know you said this problem occurred independent of the camera but let's just make an assumption in order to come up with a hypothesis then work from there: Suppose that you were winding or rewinding the film (in the camera) and the back or body of the camera vibrated open, slightly. A very thin sliver of light would penetrate and make a line across the film. If the film was moving and this hypothetical gap in the camera body was vibrating open and shut, it would cause a "strobe" effect which would imprint a series of roughly parallel lines on the film.
Now, extend this metaphor. It doesn't necessarily have to happen inside a camera. It could occur during manufacture, somehow. It could occur as you are bulk loading film from spool to cartridge if the cover of the loader vibrates open. It could occur if there is a gap in the lips of the film cartridge, either in storage or handling, before or after being shot. It could also occur while processing. Let's say the developing canister has a small crack in it.
What is the distance between these sets of marks? If you coil the film up so that the marks line up, does the diameter of the coil match the diameter of your developing reels, the film cartridge or the bulk spool? Determining this could help you narrow down the location where the problem occurred.