Some interesting articles in Uganda's The Independent about General Kayumba's defection. They trace his falling out with Kagame to 2000, when he was chief of staff of the army and - according to RPF officers - organized a training course for himself in England without asking for Kagame's permission and in the middle of their counterinsurgency campaign against the FDLR. When he got back from England and was the head of the National Security Service, Kayumba allegedly began to recruit a power base amongst senior RPF officers, which he continued when he was named ambassador to India in 2004.
The author could not interview Kayumba for the article, but his version of events was apparently that Kagame had begun to monopolize power and had abandoned the ideals of the rebellion. In this interview, he states some of the reasons he left, which he expands on in this article.