Why did John go to the isle Patmos?

John was exiled to the Isle of Patmos as a prisoner near the end of Emperor Domitian's reign in his old age . It was at Patmos that John wrote the book of Revelation the last book in The Bible.

Rev 1:1-3, 9 [v.1] The Revelation of Jesus Christ, which God gave him to show unto his servants, even the things which must shortly come to pass: and he sent and signified it by his angel unto his servant John; [v.2] who bare witness of the Word of God, and of the testimony of Jesus Christ, even of all things that he saw. [v.3] Blessed is he that readeth, and they that hear the Words of the prophecy, and keep the things that are written therein: for the time is at hand. [v.9] I John, your brother and partaker with you in tribulation and kingdom and patience which are in Jesus, was in the isle that is called Patmos,for the Word of God and the testimony of Jesus.