Answer: Formed boundaries regardless of ethnicity.
Step-by-step explanation:
In the Conference of Berlin in the late 1800s, the European powers divided up in Africa to their areas of influence. They did so without caring for whether ethnic groups would be divided or not, they simply wanted more land.
This is why a lot of African borders are straight.
The colonies they drew up then continued after independence and led to a situation where multiple tribes and language groups are inside a country whilst some are separated. This has led to tribal conflicts that have claimed the lives of millions since independence.