What led to the fall of the Songhai empire?

They had collapsed after al-mansur had sent an army to attack them and gain trade route access. When they were defeated in the Battle of Tondibi they had soon collapsed. Plus they had a recent civil war, so they were weakened.

The Songhai Empire failed due to Moroccan invasion who wanted to control the trans-Saharan trade routes, Sultan Ahmed el-Mansoor sent an army force, which met the bulk of the Songhai army living near Tondibi in the year 1591 and vanquished them leading the fall of Songhai empire.