The biggest single cause of the disintegration of the society of Umuofia is the advent of Christianity and the way that this foreign religion ends up splitting the society in two and causing conflict in a culture which previously had been united and had clear guidelines and rules. Note how this is highlighted in the speech that a member of the umunna gives at the feast that Okonkwo hosts at the end of Chapter 19:
But I fear for you young people because you do not understand how strong is the bond of kinship. You do not know what it is to speak with one voice. And what is the result? An abominable religion has settled among you. A man can now leave his father and his brothers. He can curse the gods of his fathers and his ancestors, like a hunter's dog that suddenly goes mad and turns of his master. I fear for you; I fear for the clan.
The book describes very clearly what happens when Christianity arrives and how its first converts are those who were traditionally excluded from Igbo society, and the tension that this causes. This conflict is heightened however when traditional taboos are openly challenged and violated, and also with the arrival of education and how this serves to drive a wedge between the Christians and those who have not ascribed to this new religion. The entire book clearly displays that it is the missionary endeavours of Westerners that initiate the disintegration of this rich and vibrant pre-colonial society.