This practice reportedly goes back to a group of volunteers, 'Mikuni Hoshidan((Volunteers for Imperial Japan) who were distressed at the sorry state of the Palace immediately after the defeat in 1945.
1945年の敗戦直後に有志「皇国（みくに）奉仕団」が皇居の荒れ様に心を痛めて始めたのが興りといわれる。