I do not know when it started but I just began to notice it a while ago. Subway subs have the meat of a cold cut combo on top of the cheese and extras. The local drive-in I sometimes go to does that with their fish and chicken sandwiches. The country club I ate lunch at as a guest of a member had a po' boy that they put the coleslaw under the BBQ beef.
When did it become common to put the lettuce, onions, tomatoes and other stuff under the meat/poultry/fish of a sandwich?