It's combination of multiple factors. I think education has the most pronounced effects on fertility/birth rate, though.

https://blogs.worldbank.org/health/female-education-and-chil... https://rdcu.be/c7r33

There's also papers on extreme working hours having negative effects on fertility: https://www.aijssnet.com/journals/Vol_5_No_1_February_2016/4...

I don't find it surprising since extreme work hours --> worse health/social outcomes --> low fertility.

China and Japan has similar thing going on, but I'm sure there's way more nuance to this issue than my statement shows.