Oddly enough, I just was reading about this in a book about coffee. The reason for the uprising was colonization. Countries enduring this form of rule usually see infrastructure and industry that benefit the colonizers, not the people native to the country. For instance, roads tend to be built from major cities to the transportation hub (airport, rail, whatever) that leaves the country, not to outlying, smaller towns.
In Kenya, farming was done by a tenant system, benefiting the British but giving the farmers little right to the crops on land they had once owned. The amount of land the farmers were allocated kept being reduced while the hours increased and conditions of labor grew worse. The book I'm reading claims locals were forced to "climb into vats of fermenting coffee beans to loosen the mucilage with their feet." The Kenyans were seen as cheap labor for the production of British commodities.
Predictably, resentment increased and civil disobedience blossomed. Retaliations against suspected Mau Maus increased, resistance increased - the spiral grew until war was declared.
This is only a broad generalization of the causes, created from my memory of the African perspective and refreshed by wiki and the book I'm reading.