There was not really one event that brought this case to the Supreme Court. Instead, the case came to the Court because of the general trend of the time, which was towards more government regulation of businesses.
This case was one of the first antitrust cases to be brought under the Sherman Anti-Trust Act of 1890. It was part of the overall Populist/Progressive effort to fight against monopolies. The case started out with President Theodore Roosevelt ordering his Justice Department to bring suit against the Northern Securities Corporation. This was the event that started it on its way to the Supreme Court, but the case made it to the Court more as part of the general trends of the Progressive Era.