Economically It wasn't that much of a fad to let go of Africa, it was filled with massive resource pools and cheap labour to harvest it.

The pressure for decolonisation was mostly political and racial. During and after WW2 the United States took on a policy of anti-colonialism and had basically excluded the colonial empires from mattering in world politics as much as before. Roosevelt had seen colonialism as the new threat. Furthermore the new spread of Communism by the Soviet Union threatened integrity of colonial rule as Communism was greatly against such institutions.