It is presumably old age, however only Natsume could tell you that for sure.
Nina Kulagina died in 1990.
Nina Broe died in 1993.
Nina Coltart died in 1997.
Nina Boyle died in 1943.
Nina Grieg died in 1935.
Nina Yefimova died in 1996.
Nina Clifford died in 1929.
Nina Kosterina died in 1941.
Nina Dumbadze died in 1983.
Nina Savina died in 1965.
Nina Makarova died in 1976.
Nina Catach died in 1997.