Actually there are a lot of reasons for dead people apparently coming back. Burying bodies so they stay down is actually a big effort for most cultures, wild animals and weather conditions can easily pull a body from a shallower grave and redeposit it somewhere else. Add that to the tendency of bodies to continue to change after death, or at least appear to (gums receding looks like teeth growing, etc.) and it's not hard to see how people in earlier culture s might get the impression that the dead are up and walking around. Factor in also how diseases can take members of a family or others close to the dead person invisibly, and, well...