The main reasons for Okonkwo's downfall are his reluctance to accept change and his desire to be the absolute opposite of his father. He refuses to accept any kind of change in this novel, the most obvious example being with the white missionaries. He refuses to accept any of the changes that the missionaries bring to Umuofia. He reacts to adveristy (after his exile) the same way he always had - with violence. As for his desire to NOT be like his father, he will do anything to prevent others from seeing him as weak. The most obvious example of this is his killing of Ikemefuna. He was told not to participate in this killing, yet he felt he would be seen as weak if he didn't. This shows he has no balance between the masculine and feminine in his life. Once he kills Ikemefuna, his life spirals downward to his ultimate death.
In my opinion, the reason for Okonkwo's downfall is that he is unable to accept change. He wants his family to remain strong throughout the clan, and definitely does not want that to change just because these white men came to their village. Throughout his life, Okonkwo has always been accustomed to the Ibo way, and his inability to adapt to a new way makes Okonkwo's life to fall apart.