Prior to Oedipus' arrival, Creon was left the thrown while the true king, Oedipus' father, went to consult the oracle for advice. Of course when doing this he was killed, which set into motion Oedipus' arrival, unseating of Creon and marrying of his mother. The question then is, in this story, why is it that Oedipus as king becomes a fool when his father, as king, went to seek advice.

What can be gained from this story is that in order to become king one must take something from somebody else and is thus thrust into a life of always being on the defense. In such a state, one trust nobody as everyone is potentially after your power. In other words, power corrupts in a manner of paranoia. Thus, it lead Oedipus'...

It led Oedipus to laying false accusations and ignoring the advice of experts. and, in the end, this process of being king actually led to the downfall of all involved men. Thus, although wisdom led them to a position of power, it was also the cause of their fall.
To return to the question of what is it about being king that makes even a wise man foolish - according to the lesson of Sophocles, in his story of Oedipus, is that all it takes is another wise man to come along and steal one's power out from under them.

Bibliography

Grene, D. And Lattimore, R. Sophocles I. Chicago: University of Chicago Press.