Why does water vapor try to move into the wall cavity?
Water vapor always moves from warm air towards colder air because warm air holds more water vapor (and therefore has higher vapor pressure). Nature always tries to equalize pressure differences. So in the winter, water vapor from the warm interior of a home tries to move toward the colder exterior. Then in summer it reverses, and water vapor moves from the hot, muggy exterior to the cool, dehumidified interior of the air-conditioned home.