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.