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Death by unnatural causes often means there was foul play, or something wrong was done that resulted in the person's death. This is as opposed to natural causes which include things like heart attacks or other health-related phenomena.
quatt47
It means that they have died from an external cause, an accident, murder, in a fire, poisoning, drowning for example.
John
natural causes are things like disease or old age that cause the person to die from something to do with their own body. unnatural causes would be things like murder or accidents where an external force kills the person.
Anonymous
The person was murdered, or killed accidentally, or committed suicide. The person did not die of a disease that he contracted naturally.