The book is called Things Fall Apart because it suits the book. Many things which are expected to last a long time suddenly fall apart due to circumstances. Okonkwo's life falls apart, as does the life of traditional Nigerians.
It means that Things Fall Apart. This implies first, that there are things, and then states that they fall apart. In terms of it's relation to the content of the book, it highlights many things, the reasons they fall apart, and how they fall apart, in many intricate details.