The answer to this question is, to some degree, a matter of opinion. We do not know with certainty which events and leaders were most important in causing Africa to decolonize. Some people might give more credit to African leaders such as Kwame Nkrumah while others might point more to Western leaders. My own view is that it was events, much more than individual leaders, which led to decolonization in Africa.
In my view, the major events that led to decolonization were World War II and the Cold War. The war led to decolonization mostly because it severely weakened the colonial powers. The main colonial powers in Africa were France and the United Kingdom. These two countries were on the winning side in WWII, but both were badly damaged by the war. France was occupied and was fought over while the UK had to spend tremendous amounts of money to maintain its fighting forces. These factors left the two countries without enough resources to continue to hold their African colonies.
The war also caused the United States to become the dominant power in the free world. The US was not hurt by the war in the way that France and England were. It emerged as the undisputed leader of the free world and was soon embroiled in the Cold War against the Soviet Union. The US wanted decolonization for at least two reasons. First, the US was traditionally opposed to imperialism on ideological grounds. This was particularly true of Franklin D. Roosevelt and his successors. Second, the US felt that decolonization would help win the support of Third World countries in the Cold War.
I would argue that WWII and the Cold War were what drove decolonization in Africa. I believe that individual leaders such as Nkrumah and Kenyatta were not nearly as important as the great events that made decolonization more likely.