Nature could kill them
Nature knows a million ways to kill someone. Emigrants on the Oregon Trail had to watch their backs all the time. Animals could trample their owners. A tornado might touch down right on their covered wagon, scattering everything they needed to survive. Rattlesnakes could decide to see what an unsuspecting emigrant tasted like. Livestock stampedes could leave emigrants broken, squished, and dead. When fording rivers, oxen could panic, overturning wagons and killing their occupants. Lice infestations could make life miserable for people who were already uncomfortable due to having to ration food and water supplies. Trees might drop branches on their wagons, or on their heads.