In general: an unhealthy lifestyle. In the relevant context the overwhelming majority of people die of diseases when relatively old (and not accidents or violence).

Other than that: suicide is often underrated - it is often the main non-disease cause of death for males.

(Eg when checking the stats for my own demographic I had about a 6.55% risk of death the next 30 years. Out of this some 10% were from suicide, 5% from accidents and only 1.5% from infectious diseases but over 50% from malignant neoplasms and circulatory system diseases.)

Useful statistics: http://www.deathriskrankings.com/default.aspx?AspxAutoDetectCookieSupport=1 Some useful tips: http://longevity.about.com/od/liveto100/tp/Avoid-The-Top-Five-Killers-Of-Older-Men.htm