Why was Alexander Selkirk cast away on the island of "Mas a tierra" in 1704?

spent four years and four months as a castaway (1704-1709) after being marooned by his captain, initially at his request, on an uninhabited island in the South Pacific Ocean. He survived that ordeal but died from tropical illness years later while serving as a Lieutenant aboard HMS Weymouth off West Africa.

one of the sources of inspiration for the writer Daniel Defoe's fictional character Robinson Crusoe.