1Nature regarded as a (manifold) source of causation; the regular and predictable causes of a thing.
2Old age or disease as causes of death, as distinguished from accident, murder, etc.
natural causes/ˌnatʃ(ə)rəl ˈkɔːzɪz/ /ˌnatʃ(ə)r(ə)l ˈkɔːzɪz/
Mid 16th century; earliest use found in Image of Idlenesse. From natural + the plural of cause.