I too have experienced such "deterministic" hallucinations. This has fascinated me for some time.

One theory is that the phenomenon is always there but that that visual layer is tuned out, just how with practice you can tune into many visual layers: you can learn to see floaters, the white blood cells moving through the capillaries in your retina, your own heartbeat, noise in the image, etc.

It may serve some function yet to be discovered. At the very least, it is beautiful.