What authority did the Supreme Court cite for taking such a role in expanding the scope of the Constitution?
I think that you might want to examine the basic concept of Judicial Review. Established in the case of Marbury v. Madison, Chief Justice John Marshall was able to assert the premise that the Supreme Court had the authority to interpret the Constitution as it saw fit. The idea of judicial review helped to establish the Supreme Court as being the primary authority that is able to effectively understand how its scope can be altered and why it should be altered, if it should be, at all. In the case, Marshall was able to craft a decision that argued that the Supreme Court held greater authority than lower courts. At the same time, Marshall was able to clearly argue that the Constitutionality of laws and actions rested with the Supreme Court, something that had not been argued at the time. In this, the authority of the Supreme Court was established. It is from this starting point where the court was able to be charged with the responsibility of expanding the scope and reach of the Constitution, something that had not been previously envisioned before the decision.