Foreshadowing.

Mal has the crew disguise Serenity as a Reaver ship and they travel to Miranda undetected. They find its 30 million colonists dead, and a recording that explains an experimental chemical to suppress aggression had been added into Miranda's atmosphere. The population became so docile they stopped performing all activities of daily living and placidly died. A small proportion of them had the opposite reaction and became insanely aggressive and violent. The Alliance had created the Reavers and this was the secret in River's subconscious.

Need I say more? I could, ya know.