Slavery already existed in Africa before the arrival of Europeans on the continent, but slavery became a lucrative business for both slaving Africans and slaving Europeans. The accentuation of slavery in Africa came about because slave sales to America became a lucrative activity.
People became enslaved in Africa mainly by wars.
Another form of slavery present in Africa was debt slavery: the indebted individual became a slave to the debt creditor.
Slaves were seen as cheap labor for Europeans which made them a big target.