There were two main reasons for the move to militancy.
The first reason was very deep-seated and rooted in history. This was the fact that African Americans had been oppressed for so long. Naturally, this history led to a great deal of anger on the part of some African Americans. For them, the history of injustice called for a more aggressive and angry response than Martin Luther King, Jr. was willing to accept. They therefore turned to militancy.
The second reason was caused by more recent events. In 1964 and 1965, the Civil Rights Movement had won its greatest victories. These victories gave African Americans legal equality and protected their right to vote. The problem was that these victories did nothing to help alleviate poverty and other such problems in the short term. African Americans felt as if they had waited a long time and worked very hard for rights and that those rights had not benefitted them in any tangible ways. For this reason, they turned to militancy in the hope that it would bring about more material benefits than King’s methods had.