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

As others have pointed out, this is posterization, where the tonal resolution is lower than what the eye is capable of perceiving.  Looking at the image data for the highest-quality file available for us to download from dpReview, it's only 740Kbytes, in other words a very low-quality JPEG.

One of the compression methods used for JPEG files, is to limit the number of tones/hues in the color palette.  When this tonal down-sampling is performed, lower priority is given to large areas with low tonal detail, i.e., smooth backgrounds and skies - hence the problem exhibited here.

Your first step should be to output the file from your converter as the highest-quality JPEG that it can produce.  This should be many Mbytes.  If this fails to correct the problem, the next step is to examine the original RAW file.  I'd be happy to have a look at that for you, if you can make it available somewhere.

--
Qualities possessed by God in infinite proportion: Love, Grace, Power, Righteousness, Wisdom, . . .
Qualities possessed by humans in infinite proportion: Ignorance.
- Marianne