Precursors decided that humanity would inherit the Mantle, and the Forerunners believed that they were going to be erased like many other species who failed the Precursors' 'test' supposedly had. They struck out first and drove the Precursors out of the galaxy. The Precursors were in awe at the capacity for violence that their creations had and chose not to respond in order to experience the full power of what they had created, but they also sought survival strategies too - some Precursors disappeared and went into suspended animation (we do not know what happened to them), but others became a fine powder stored on ancient automated starships. The powder was designed to regenerate their old forms, but as 10 million years passed the powder became defective and brought only sickness and disease- culminating in the birth of the Flood.