The immediate cause of Singer's becoming an actor was acquiring an interest in the theater.
In the chronology of his biography, it is indeed the event which directly precedes becoming a theater actor. We are told that Singer "acquired an interest in the theater," and the very next sentence starts with his stage name, Isaac Merritt, and his trip to Rochester to "become an actor." This makes sense because attempting to become a professional in an area that interests you is a logical path to take.