Evans, Sara M. Personal Politics: The Roots of Women's Liberation in the Civil Rights Movement and the New Left. New York: Knopf : distributed by Random House, 1979.
Women's experience in the Civil Rights movement as well as the New Left catalyzed and sharpened their political stance. Quickly, though, disillusionment set in as even within these groups they found themselves under-appreciated, minimized, and disregarded. Thus they were motivated to join forces and start their own cause.
Analogy: women in the 1830s-40s abolition movement were motivated to start the first wave feminism.
Evidence: extensive oral history by an insider to the movement.