The cause for the gradient is a weakening of X-ray radiation at its origin in the X-ray tube. A closer look at the phenomenon can be found in my FAQ. This effect of variable recording of photons phycisists call „anode heel effect“.
As part of my creative process I rotated the composition shown above by 180 degrees and exposed it a second time with the same parameters. Note that post-production as well was done equally for both X-ray exposures !