I'm just quoting from a website
"the factors leading to the downfall of empires of Ghana, Mali, and Songhai
Climate change and struggles with Berber groups in the Sahara led to the downfall of the Ghanaian Kingdom
Later kings followed the patrilineal tradition, where the eldest son succeeds the father. Since there seems to have been no strict tradition, there were many fights over succession. This led to civil wars and was a large part of the reason why the Mali Empire declined.
Like the Mali Empire, the Songhai Empire also suffered from many battles over succession. In Songhai, the brothers of the king often tried to depose him; in turn, some kings tried to protect themselves by killing all their brothers. The last great battle for succession occurred when a great army general challenged a new king. Many men were killed in the battle for control of the country and the Songhai army was weakened. When troops from Morocco invaded to seize control of and revive the trans-Saharan trade in gold, the Songhai Empire could not win and was conquered, making this the last of the great West African empires."
