During the Communist witch hunt...
During the Communist witch hunt, led by Senator Joe McCarthy, many individuals in the film industry were falsely classified as Communists. In so doing, they could no longer work. In order to continue in their profession, they hid from detection by using pseudonyms.
When Kirk Douglas was acting in the film production of Spartacus, the story of an escaped Roman slave who rose up an army to secure independence and freedom, the screenwriter was Dalton Trumbo. But Trumbo was on the blacklist for being a Communist, so he was working under a pseudonym.