Sour lunch meat why does it happen?

why does lunch meat go sour? I thought it was filled with salt and nitrates and so on. Last Sat I bought a 1 pound of ham and a pound of chicken roll. I left them in in their palstic interleves and put them in a plastic dish and covered. I put the dish in fridge and promptly forgot about it. tonight I was cleaning out the friidge and opened the lid and you could smell the faint stink of sour meats. How does this happen with lunch meat? OK 9 days but still in a covered dish and not opened at all and refidged. salt and nitrates....WTF?