Almost certainly this. Not just chicken but breading ingredients, the oil they fry them in, probably salt content. Maybe sauces too? Oil sounds like the likely

Almost certainly this. Not just chicken but breading ingredients, the oil they fry them in, probably salt content. Maybe sauces too? Oil sounds like the likely culprit to me. About a decade ago a bunch of cities, including Chicago, started regulating the types of oil that could be used for frying in restaurants.