what causes Oedipus the king to murder his father and marry his mother?

The story goes something like this:

King Laius, like most kings, really wanted a boy. Laius had been married to Jocasta for some time with no child. Laius consulted the Oracle of Apollo at Delphi. The oracle tells Laius that if he does have a son, his son will kill him. Laius, of course, gets a son. Not wanting the prophesy to come true, he sends the baby to be abandoned on some mountain. A shepherd saves the baby and brings him to king of Corinth and his queen, Merope. They adopt and raise him.Oedipus finds out he was adopted and consults the same Oracle who tells him the same thing save the part about his real birth parents. Not wanting to kill his dad, Oedipus travels to Thebes. On his way he encounters a travelling caravan. Both claim right of way on the path. There is some major road rage or rather path rage and Oedipus kills the man who turns out to be his birth father. Oedipus makes it to Thebes, solves a riddle from a nasty Sphinx and is a major hero. Oedipus marries the queen, now a single lady and on the market. After many years of marriage and four children (Yuck!) Oedipus finds out Jocasta is actually his mother. Like any self respecting dude who finds out he has inadvertently scored with his mother, Oedipus gouges his eyes out and moves to a mountain. That's the really abridged version!

One can go deeper to find the answer to this question. Long ago, one of the members of the House of Cadmus kidnapped a boy. This angered the gods, so in Greek fashion they decided to mete out punishment on the descendants of the guilty man. Oedipus was fated to kill his father and marry his mother. There was nothing anyone could do to avoid this. Laius and Jocasta tried to kill Oedipus, but that failed; Oedipus tried to run away from the people he thought were his parents . . . and that led him straight to the murder of Laius. It was his destiny. A terrible destiny indeed, but Oedipus became a respected prophet after he blinded himself. The House of Cadmus was cleansed at last.

Point Blank: Oedipus had a curse on him that he didn't know about. Forcing him to kill his father (He doesnt know in the beginning that he did) and unknowingly have sexual intercourse with his mother.