I do this when eating food I don't like and I used to be a very fussy eater. I was at my friends house and she cooked food and I was forcing it down to be polite and she asked if I liked the food. I said yeah its really good and she burst out laughing because apparently my face looked like I was eating a war head. She said I can tell when you don't like something because you always scrunch your face. Apparently I used to do it in conversations as well.