Why did american cities experience explosive growth in the late 19th century?

"There were 2 main reasons:

The Second Industrial Revolution created the need for a workforce in new factories, so many people moved from the countryside to the cities to find jobs in these factories. The effects of the depression of 1873-1896, which spread to America, further encouraged people to go to the cities to find work.

There was a long economic depression in Europe from 1873-1896, which encouraged many Europeans to emigrate to America. Most of these arrived at major port cities, and found jobs in factories so stayed there.