The drawing of the boundaries in the manner of the interest of the European colonial powers resulted in very big problems once the African countries were granted independence.
The problems were mostly because people from the same ethnic groups were separated by borders, but were in the same ones with people from other ethnic groups, usually historical rivals with whom they had bad relations. Also, the religious factor had a big influence, as there were multiple countries were some parts were predominantly of one religion, and other parts of other religion, and that brought in even more tensions.
That has resulted in instability, constant tensions, civil wars, terrorist organizations, separatist movements, genocides, pretty much everything that is not supposed to happen for a country to prosper.