This u-shaped curve is present in other countries too, e.g. the U.S.
It's clear to me that modernity subdues the desire to reproduce in most people, and the availability of contraception and abortion makes it easy to have sex while avoiding pregnancy. As fewer people have children, we see the gradual collapse of the social infrastructure that both supports parents and also cajoles young adults into becoming parents. Having children comes with trade offs, especially financial, and most modern people just don't care enough about having children to tolerate those trade offs, especially when it comes to having big families.
Three things seem to inoculate against this:
1. Having an unusually strong desire to have children, and a willingness to tolerate the trade offs, usually motivated by a sense of spiritual purpose (
@CRPakaluk's book is superb on this).
2. Being a member of a religious subculture that has preserved its pro-natal social infrastructure.
3. Being either so rich that the financial trade offs don't matter very much, or so poor that the welfare state will absorb those trade offs on your behalf. That seems to be what's happening with the u-shaped chart.