How did raiden in Metal Gear Solid become ninja

During the SOP (Sons of the Patriots), Olga stated to Raiden that the life of her child (Later on known as Sunny) depended on his own. It is known that after SOP, Raiden had complications with his girlfriend of the time; Rose. After Rose had told Raiden his child (that she admitted she was carrying at Arsenal Gear in MGS2) was miscarried. Through hearing this and the fact that Rose and the Colonel had gotten married, Raiden left to Alaska where the Patriots found him and severed his head.

His arrival in MGS4 surely shocked Solid Snake and everyone else as well, as they had thought he too had died and was dead for good. Raiden's head-severing was just the start of him becoming a cyborg ninja.

(This also explains why he wanted to protect Sunny so much.)