Life expectancy is determined by a number of factors but in Japan, life expectancy was the shortest among the G7 countries until relatively recently (1960s) and so the sudden increase in life expectancy is likely to be due to changes in lifestyle in Japan.
One major factor is diet. In Japan, consumption of red meat is relatively low which means that saturated fats are a small proportion of the diet. Consumption of fish is incredibly high in Japan which means that polyunsaturated fats are commonly consumed. Moreover, Japan has a fairly diverse and high protein diet including legumes such as soybeans and a variety of seaweeds. On average, Japan tends to have a relatively plant based diet with lower levels of processed food.
However, the increasing move towards big cities and more urbanised living is resulting in changes to diet. For example, more people regularly visit fast food restaurants and eat out after work. These changes could contribute to shortening life spans in Japan.