Essentially, modern medicine and science didn’t have proper explanations for an improper burial or the methods of disposing of bodies. According to Vampires, Burial & Death by Paul Barber: animal scavenging, shallow graves and the erosion of soil, grave robbers, and flooding were all ways in which the deceased would “unearth” and return to this world.
1. Animal scavenging: dogs and wolves were thought to be the natural enemies of vampires (ie. we’re witnessing Twilight in the past), and thus when an animal of that nature began digging up and unburying the dead, they were seen as attacking literal vampires rather than just scavenging for food. Because the animals would have unburied the dead prior to its initial decomposition, it only heightened the mythology of folkloric vampire beliefs.
2. Shallow graves and erosion of soil: since funerary practices weren’t exactly the most modernized way of burying bodies at the time, many bodies would basically erode back onto the surface. Many would witness this as a sign that demonized souls were returning to earth, and thus they would freak out about the bodies of the now undead being possessed.
3. Grave robbers: If animals aren’t scavenging, then people are. In search of goods or body parts (the belief in magical sorcery, etc. was still prevalent during this time period), or they would be upset that certain people with “unclean” legacies were given a proper burial. Bodies would be unearthed and, in return, the mythos would continue.
4. Flooding: oftentimes, flooding would wash out coffins and leave the dead above ground.
Of these four experiences, local townsmen would rarely actually observe the natural phenomenon, and would in turn play the guilt card onto supernatural concepts. What could not be explained outright had to be a supernatural idea, and we can witness this throughout most folkloric stories concerning vampires and zombies.