I know that if you choose "Delete Cropped Pixels" in the crop tool, that it will preserve the background layer, as a background layer. However, I wished that I could still maintain this function when cropping without the delete cropped pixels option checked as well. It seems like a silly thing to have to recreate that layer as a background manually every time. Is there a script out there that can do this?
The reasoning behind this is that the background layer is a special layer that must be the size of the canvas (i.e. the pixel dimensions).
Other layers can have pixel extents that are larger than the defined canvas size. I believe that Adobe calls this "big data."
Since you are opting to not delete pixels when cropping, then the background layer needs to be converted to a "normal" layer in order to preserve the pixel data.