The British, actually, were a little more deliberate than the above answer suggests. They did pay attention to which tribes lay within the boundaries they selected for their colonies. Through a very successful and ingenious process they called indirect rule, they found minority tribes that had been largely oppressed by majority tribes in the past and they empowered them. They gave them education, jobs, positions in government, and most importantly, military training. The Belgians copied this model, as did the French to some degree. In exchange for the privileges, the minority tribes were loyal to the empire and maintained the steady flow of resources to the mother country.
When colonialism ended, the same tribal resentments existed, but now one side was armed and trained, while the other side had numbers and a grudge. It has been a recipe for disaster in post-colonial Africa.