The answer is: He was accused of disloyalty and lost his security clearance.
This happen during the period when general public were anxious that a lot of spies from the soviet union have invaded the united states (red scarce).
At that time, the opposition of Robert Oppenheimer accused him of disloyalty and said that he's a part of Soviet union's spies that want to limit united states' technological development. Even though all Oppenheimer want is to stopped mass killings from hydrogen bomb to ever happen gain.
