"The SDT began emerging in the West after World War II. As societies became richer and goods cheaper and more plentiful, people no longer had to rely on traditional families to afford basic needs like food and shelter. They could look up the Maslovian ladder toward “post-material” goods: self-fulfillment, exotic and erotic experiences, expressive work, education."

I think there's one more cause for the Second Demographic Transition: The majority of jobs are now in the service sector. After spending the day negotiating and putting up with others, people may want some solitude.