He may be angry and throwing the book may be a part of that. But I can't see angry. Most behavior does not just randomly happen. It occurs in a context. Some behavior preceded it and some behavior followed it.
Behaviorists are persons who study the behavior of others.
A behaviorist would be very concerned with the behavior that preceded the behavior of the child - this is called the Antecedent . The behaviorist would objectively describe this antecedent behavior as well as the behavior of the person being studied. The Behavior was Billy throwing a book across the room. A behaviorist would not necessarily conclude that what the teacher said caused Billy to throw the book. At this point we are just looking at the facts of the situation.