What caused this demographic switch ?

The argument is that the mortality transition occurs exogenously and is the cause of a subsequent decline in fertility and rise in urbanization. This exogenous demographic process plays out over time, and has effects on economic, social, and political development. Hope this helps. :)

in the 1800s, chinese workers migrated to the united states, first to work in the gold mines, but also to take agricultural jobs, and factory work, especially in the garment industry. chinese immigrants were particularly instrumental in building railroads in the american west, and as chinese laborers grew successful in the united states, a number of them became entrepreneurs in their own right. as the numbers of chinese laborers increased, so did the strength of anti-chinese sentiment among other workers in the american economy. this finally resulted in legislation that aimed to limit future immigration of chinese workers to the united states, and threatened to sour diplomatic relations between the united states and china.

i believe it would be a single seller : )