This is largely due to China's population dynamics.
Here I've made a chart that shows the number of births (and deaths) in China over the last 69 years: https://ourworldindata.org/grapher/births-and-deaths-project... 30 and also 50 years ago China had large cohorts of newborns, these are the 'ripples' you see going through the global population pyramid.
The number of births is always determined by two factors: the number of children per woman in the reproductive age bracket (called the Total Fertility Rate). And the number in the reproductive age bracket.
Here is the Total Fertility Rate for China: https://ourworldindata.org/grapher/children-per-woman-un?tab...
The large cohort born around 50 years ago is the parent generation of the large cohort born 30 years ago.
So the question is what explains the large cohort in the 1960s? That is the cohort born right before the rapid reduction of the Total Fertility Rate (from 6.4 children per woman in 1965 to half of that 15 years later as the chart above shows; this is largely before the introduction of the strict one-child-policy by the way). And it is the cohort born after one of the largest mass deaths in history. The Great Leap Forward Famine from around 1951 to 61 https://ourworldindata.org/famines#great-leap-forward-famine... Around 30 million died and as in all famines the fertility rate declined substantially, to then jump again right after the famine.
Good questions! I should have mentioned them in the post. Let me know if you want to know more.