It's from nitrate curing. Ham, bacon, mamy sausages, and most lunch meats go through this process. Sometimes the chemical is added directly via saltpeter, and sometimes indirectly via stuff like celery juice.

From the statement in Lancet Oncology, "Carcinogenicity of consumption of red and processed meat":

"Processed meat refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation"