You’re probably extending your neck forward as you apply mascara (and contacts for some, but not all, people) Opening your mouth may be more comfortable for the muscles at the front/sides of your neck in that position. As you note, it wouldn’t be a major effect, but since your attention is focused elsewhere it may be semi-instinctual at first, and pass into habit over a period of years
Just a WAG. It does explain why not all contact wearers do this, though, since there are several distinct technique to apply contacts.