Why was the Roman Emperor Claudius imprisoned?

Emperor Claudius was not imprisoned. You must have got confused with Tiberius Claudius Narcissus. He was a freedman who was in the inner circle of the court of emperor Claudius. He was imprisoned and executed by Agrippina (the emperor's last wife) soon after the emperor's death. Ancient writer Tacitus said that Agrippina had him executed because he supported Britannicus (the son the emperor had with a previous wife) for the imperial succession against Nero (Agrippina's son) and plotted to bring Agrippina down and, though her her downfall, Nero.