Tiresias replies that he is beholden to no one but Apollo—the “higher authority” that the Greeks thought was God. This allows him to speak the painful truth to Oedipus. What caused Tiresias to change his mind and speak the truth?

Tiresias really doesn't want to tell Oedipus the truth. Patricide was a huge issue, he did not want to be involved with. Besides, the future was already set so his magic intel. isn't going to change much. Still Oedipus, who has a temper, pesters Tiresias until he spills the truth.