perhaps they aren't comfortable with showing their true opinions expressed through their facials since they are so used to lying.
or they don't want to attract unwanted attention from subhuman males when their mouth widens.
normies would argue that it's just a form of politeness similar to when you yawn but i have doubts. that could only with certainty be said for males since they are the only ones for whom social conventions apply to. women are wholly sexual beings, their entire life circles around nothing but sexuality and relationships are but an extension of that.
so i'd assume since their entire social outward image is a lie that when something takes them by surprise they have to react in order to hide their real character from showing. in other words an attempt to hide body language with body language.