For some reason the body essentially attacks itself and destroys the cell in the pancreas gland that produce insulin. A subset of the white blood cell population the cytotoxic T cell has been observed to kill the cells in the pancreas.
Perhaps a particular infection with a bacteria or virus causes the immune system of genetically predisposed individuals to mount an immune respons against the insulin producing cells, in much the same manner as infections with certain strains of streptococcal bacteria leads to rheumatic fever.