Why was colonization the cause of so much violence in africa in the 20th century?

The Berlin Conference set down rules for dividing up Africa. This meant that the European colonizers established national borders that grouped certain African ethnic groups together. This meant that despite ethnic groups being historical enemies, they were now being forced to live within the same boundaries. 

Ex: German & Belgian colonial governments aggravated the historically tense relations between the Hutu and Tutsi ethnic groups in present-day Rwanda and Burundi. The ethnic violence between these two groups resulted in a war that led to the deaths of hundreds of thousands of people (as we saw in the movie Hotel Rwanda).