What happened to my photo? (background is pixelated?)

The background has so little light that the total amount of light available as the function of distance from the background light source varies on the order of 1 count in the 14-bit ADC of the sensor. As the result the color representation of the background looks like a step function, not like a smooth curve as a function of distance from the light source. These steps is what is giving you the stripes of shades.