People attribute it to empowerment of women, but I wonder if it's more correlation than causation. Women empowerment happened in the same time frame there was a large shift towards urbanization. The situation across the world before was like ~80% of people living in rural areas, and ~20% living in cities. Now those proportions are approximately flipped in many places. IIRC cities appear to be a net population sink for most of history, counting on an steady stream of people moving from the countryside each generation to replenish sub-replacement numbers. Raising children "free-ranging" is more straightforward in the countryside. In cities they demand a lot of micromanagement and resources from parents, because car-infested, cramped urban landscape is expensive and hostile to children. So perhaps the causation arrow flows from accelerated urbanization to both women empowerment and sub-replacement fertility rates, not necessarily from women empowerment to sub-replacement rates.