What causes this?
This effect is called solarization and has been observed since 1840. Photographic emulsions that show solarization do not saturate with increasing exposure, but return to appear unexposed.
Silver based photography
The light sensitive content of traditional photographic emulsions is formed by grains of crystalline silver halide. Halide means that the silver is compound with one of the halogens bromine, chlorine, fluorine, or iodine.
When light hits a grain of silver halide, the light is absorbed and its energy breaks a silver halide molekule. The halogen permeates away from the grain, and crystalline silver remains to form the latent image. The developer later reduces the remaining silver halide in those grains that were exposed and already contain some crystalline silver. This increases the amount of crystalline silver and makes the latent image visible. Finally the fixer will remove any remaining silver halides, and makes the emulsion insensitive to light.
Not all photographic emulsion show solarization. Silver chloride and silver iodine based emulsions do not show significant solarization, but emulsions containing silver bromide do. When the silver bromide grain is overexposed, the bromium escaping from the center of the grain passes through the grain surfaces and recombines with the crystalline silver thereby removing the latent image.
The details of this process are described in the Regression Theory (H. Luppo-Cramer, 1911), the Coagulation Theory (H. Arens, 1925), and the Bromide Migration Theory (H. Kieser, 1929). Today the consensus is that the Bromide Migration dominates, but the other theories also play a role.