I tried to make the point that, in the past, it was common practice to select STBY on the transponder before selecting a new code, but my post disappeared into the ether!
So what if, TOC, one pilot goes back for a comfort break (likely time for it), explosive decompression, remaining pilot, stunned by events, starts a descending turn, put transponder to STBY, selects 7700 and then goes hypoxic, having failed to make donning the mask his first action?