Lateral Thinking #24- Death In A Field -

A man lies face down in the middle of a field, dead, wearing a backpack. A few seconds before his death, the man knew he was about to die. There are no objects of importance in the field nor is the field unusual in any way. The man was not killed by a person or animal, nor did he kill himself. The man was perfectly healthy right up until the moment he died. He did not die from natural causes (e.g. old age, heart attack, health condition) nor did he die from poisoning or allergy. He was not killed by an act of God (e.g. lightning, hail). The contents of his backpack did not cause his death.

So... how DID he die?