The founders of Westminster Seminary were all former faculty at Princeton Seminary and departed Princeton over what they saw as modernist influences.
"Princeton [had] excelled under the leadership of distinguished teachers who devoted themselves vigorously and effectively to the development, propagation, and maintenance of the Reformed faith. Among those best known as teachers of the great scriptural system of theology set forth by Princeton's first professor Archibald Alexander were Charles Hodge, J. A. Alexander, B. B. Warfield, and J. Gresham Machen. But eventually a movement surfaced to end Princeton's adherence to scriptural theology, and in 1929 Princeton Theological Seminary was reorganized under modernist influences.
"Among the Princeton faculty who loved the Reformed faith were Robert Dick Wilson, J. Gresham Machen, Oswald T. Allis, and Cornelius Van Til. Almost immediately after Princeton's reorganization, these four men founded Westminster Theological Seminary in Philadelphia, and, with others who were invited to join the teaching staff, continued the exposition and defense of the Reformed faith."