What was a significant underlying cause for the initial enforcement of the Sherman Antitrust Act against labor unions instead of corporations?

The courts and the government at the time saw labor unions as “unreasonable restraints of trade” just like monopolistic corporations, so they used the Sherman Act to curb union power. Politicians wanted to protect business interests and maintain economic stability, and they believed that strong unions threatened that stability. Because unions were viewed as monopolistic in their control of labor markets, the Sherman Act was applied first to them rather than to corporations. For example, in 1902 the government prosecuted a railroad strike by a major union under the Sherman Act, treating the union’s collective bargaining as a restraint of trade. This early use of the law reflected the prevailing fear that organized labor could dominate the economy as much as a single corporation could.