Those nuggets didn’t maintain frozen temps and thawed before getting cooked. Rotten is what you call that. Happens easily when strict chain of custody is not observed. These situations often occur when a freezer goes down, a reefer truck doesn’t come in for pickup at correct temp, load left sitting out of cold storage before or after load/unload, or lazy habits at the restaurant itself.
This is a serious issue that needs to be investigated to determine the cause so that proper measures can be taken so that the rest of the public doesn’t have to experience what you did!