Then one day, all of a sudden, these villagers turned into violent savages.

They later found out it was caused by the
Las Plagas. Although they appeared fossilized, they were able to survive the long years by lying in a dormant state at the cellular level remaining in a spore-like form.

Apparently during the excavation, the villagers inhaled the spores and within their bodies the parasites became active again. This is how the Las Plagas were resurrected.