I can think of any number of situations where a time traveler might end up dead... eaten by a dinosaur, casually murdered by a human in a barbaric age (if you ever saw "Timeline" you know what I'm talking about), caught in a natural disaster they were investigating, subjected to "justice" in some brutal empire or other (Rome, Revolutionary France, the USSR), a casualty of a war they were trying to peek at, or taken out by the temporal equivalent of a carjacker (as presumably happened). I would think the time traveling historian's bosses would have taken precautions to retrieve the machine if its user died.