I think it probably depends on your climate to some extent and if there's any protective coating on the metal. I know for a fact that my Titan (my last truck) never got the seats soaked or anything like that, but when I removed the seats to have them recovered at about the 10 year mark there was a decent amount of surface rust that looked just like that on both seat pans. The metal was shiny steel just like what you show, so the only thing I can figure is it just rusted from atmospheric moisture because it doesn't have any protective coating on it.