Usually in Shakespeare's dramas, a king loses his throne, and therefore his power, through military defeat or acts of political betrayal. For example, King Duncan is murdered by Macbeth, Macbeth as King of Scotland is defeated in the war waged against him, and King Hamlet is murdered by Claudius. King Lear, however, loses his power in a different way.
In his old age, Lear decides to divide his kingdom among his daughters, with the understanding that he will retain his power. He intends to give up his responsibilities as Sovereign, while still enjoying the advantages of the position. After banishing Cordelia when she refuses to flatter him, Lear divides his kingdom between his two remaining daughters, Goneril and Regan, trusting them to honor him and care for him for the remainder of his life. Once Lear has acted to effectively divest himself of his throne, however, Goneril and Regan betray him. Lear finds himself completely powerless, without a home or place of refuge, and sinks into madness.