The most likely cause is electrical interference being picked up by the sensor/readout system at very high ISO. Community reports point to electromagnetic interference from the lens autofocus motor (Nikon SWM), and possibly from the camera’s own electronics. At high ISO, the camera is amplifying very weak signals, so small interference patterns can become visible as line noise or banding.

That explains why Nikon specifically warns about autofocus: driving the AF motor adds electrical activity near the time the image is being read. On the D4, changing aperture during live view or movie recording can create a similar effect because more internal electronics are active while the sensor is continuously operating.

The effect is usually most noticeable in dark areas, underexposed frames, or when exposure is pushed in post. Practical workarounds are the same ones Nikon suggests: use manual focus or focus lock when possible, avoid changing aperture during live view/video capture, and expose as cleanly as possible rather than brightening shadows later.