Im not sure that it has any specific cause, but there has been some decent scientific research showing that some patterns of facial expression are usually related to certain specific frames of mind, so that this emotional leakage can be used in lie detection and in assessing someone in an interview --- when their words say one thing and their face says something else, wise folks believe the facial expression.
I has an American friend, but since the concussion my memory for some details is a bit shaky --- could it have been Paul Ekman or something like that, who wrote some entertaining and illuminating books about lying based on such research