Men do it too. Folding the arms across the chest is a classic “body language” expression, in both genders, of what is often referred to as a “closed” attitude. It indicates that one is apprehensive, anxious, defensive, not open to interchange. If women do this more than men, I would suspect that it’s because they are more often the objects of unwanted advances or otherwise put in defensive situations. They’re protecting their personhood more than their mammary gear.