List the reasons why Africans enslaved each other.

First, consider the fact that Africa is, even today, tribal in nature, and that many of these tribes were enemies or at least rivals.

African slavery, though markedly less brutal than its European counterpart, occurred for a variety of reasons.  Status within the tribe in terms of wealth and power, profit in terms of dealing with Spanish and Portuguese slave traders, and then as an informal type of warfare, kidnapping your enemies as slaves was considered the equivalent of "counting coup" in Native American culture.

See the diary of Olaudah Equiano for some descriptions of his experience in African slavery, and later on the Middle Passage.