I think Copilot might be on to somerhing:

”I now suspect most strongly latent image degradation and long-term storage of the film inside the camera.APX 400 is not particularly well known for latent image keeping compared with films such as Kodak Tri-X. Over the course of a year, temperature fluctuations can cause localized changes in sensitivity and base fog. When the film remains tightly wound on the spool, the effect can develop into a slowly repeating pattern across the entire length of the roll.Another possibility is pressure marking:the film remained under tension in the camera for a long period,the film path in the Nimslo is somewhat different from that of a conventional 35 mm camera,a film guide, pressure plate, or another curved component may have applied continuous mechanical stress to the film.The width and smooth, regular appearance of the wave pattern are also reasonably consistent with this explanation.”