Its caused by the filament slowly breaking down - known as the Edison Effect.
Electrons leave the filament and end up coating the inside of the glass. Eventually, the filament gets too thin and breaks - majority of it is lining the glass.
Its mainly caused by poor processing at manufacture, impurities are left within the globe itself and cause this to happen. It also causes the vaccuum inside the globe itself to dissipate, leading to further resistance and heat, and therefore further break down of the filament. Its a cycle that eventually kills itself. These "chromed"globes will be running hotter than a new clean one
