John D. Rockefeller was called a robber baron because many people believed he used unethical business practices to amass his extraordinary wealth. The term "robber baron" was coined in the 19th century to describe a group of industrialists who were creating enormous personal fortunes. The first known use of the term occurred in 1878, according to the Merriam Webster dictionary.
Among the various men who were commonly referred to as robber barons during the second half of the 19th century, John D. Rockefeller was the most prominent. He created the largest company and most profitable company in the United States at the time. This was the Standard Oil Company, which dominated the oil industry for decades.
Rockefeller used questionable tactics to establish Standard Oil's dominance in the industry. One of the most controversial was his practice of demanding rebates from railroads. Because Standard Oil shipped such large amounts of oil by rail, Rockefeller demanded that the railroads offer him rebates, which in essence is a discounted rate. This policy gave Standard Oil a competitive advantage over other oil companies. Rockefeller's competitor's considered this practice and others he perpetrated to be unfair, which led to the him being described as a robber baron.