What factors are responsible for the tragedy of Oedipus the King ?

By definition, a tragedy is a play in which a hero experiences a tragic fall from a high position, typically as the result of a flaw within himself. The hero starts out as basically a good character and usually holds a position of power (like King, as in Oedipus the King). As a result of his tragic flaw, this hero falls from power and often, though not always, dies at the end of the play. Oedipus does not die, but he ends up blind and banished. There are two major factors in Oedipus's downfall. 

First, Oedipus sometimes exhibits qualities of pride or arrogance. This prevents him from learning the truth about his background and parentage earlier in the play. It is important to remember, though, that at the start of the play, Oedipus is seen as basically a good king. He cares about his people's concerns and takes steps to resolve the plague that is wreaking such havoc on his city. Later, though, when Tiresias suggests that Oedipus may be to blame for the plague, Oedipus lashes out and the reader/audience can see his arrogance. Oedipus also demonstrates excessive pride in his conversations with his brother-in-law Creon, whom he accuses of plotting against him with Tiresias. Oedipus's stubborn arrogance creates much of the dramatic irony of the play, as well, because he insists that he could not have been involved in the murder of Laius (which has caused the plague) without knowing that he did, in fact, murder the former king who was also his biological father. 

A second factor in Oedipus's downfall, and thus in the tragedy, is fate. In the play, various characters visit the Oracle at Delphi for guidance. The first instance occurs before the play starts. Oedipus's parents (Jocasta and Laius) learned that they would have a son who would murder his father and marry his mother. In an attempt to cheat fate, Jocasta orders a servant to tie the baby's feet together and leave  him on a mountain. What she doesn't know, though, is that the servant cannot bring himself to do this and instead gives the baby to another man, who brings him to the King and Queen of Corinth, where it is presumed he will be safe. Oedipus is raised as the Corinthian royalty's son, but as he grows up, he starts to hear rumors about his background. He visits the oracle and learns he is destined to kill his father and marry his mother. Thinking the King and Queen of Corinth are his parents, he flees. Along the way, he meets a caravan that includes the King of Thebes, Laius (his biological father) and slays the travelers because he feels they have run him off the road. Oedipus proceeds on to Thebes, where he solves the riddle of the Sphinx and becomes King of Thebes. There, he marries the recently-widowed queen, Jocasta (his biological mother). All of these events show that Oedipus's fate could not be avoided. This furthers the tragedy of the play because Oedipus's fall seems to be out of his own control.