Sam Wilson decides to stop being Captain America after a young black super hero, Rage, was falsely accused of a crime he didn’t commit. Rage was then convicted and moved to a prison that housed meta-humans. Being a super hero, he was a target of the villains in the prison and was beaten badly, resulting in him being a vegetable and kept alive by machines. Disgusted at the injustice, Sam makes a video announcing his intentions and returning the shield to Steve Rogers.
From – Captain America: Sam Wilson #21