What caused the development of the first large-scale organization of labor unions to occur in the second half of the nineteenth century

1870-1900

From the era of Reconstruction to the end of the 19th century, the United States underwent an economic transformation marked by the maturing of the industrial economy, the rapid expansion of big business, the development of large-scale agriculture, and the rise of national labor unions and industrial conflict.

An outburst of technological innovation in the late 19th century fueled this headlong economic growth. However, the accompanying rise of the American corporation and the advent of big business resulted in a concentration of the nation's productive capacities in fewer and fewer hands. Mechanization brought farming into the realm of big business as well, making the United States the world's premier food producer--a position it has never surrendered. But still the land hunger of white Americans continued unabated. This led to wars against the Native Americans of the Plains and the "second great removal" of indigenous peoples from their ancient homelands.

Indispensable to this growth and development were an unprecedented surge in immigration and urbanization after the Civil War. American society was in transition. Immigrants arriving from southern and eastern Europe, from Asia, Mexico, and Central America, were creating a new American mosaic. And the power of Anglo-Saxon Protestants--once so dominant--began to wane.

What many thought of as progress, however, others regarded with apprehension. Agricultural modernization disrupted family farms, for example, provoking the country's farmers to organize protest movements as never before. And the social problems that accompanied the nation's industrial development fueled the rise of national labor unions and unprecedented clashes between capital and labor. This discontent captured the attention of reformers and politicians who began to challenge traditional party politics through third-party movements.

https://whorulesamerica.ucsc.edu/power/history_of_labor_unions.html