It may have been embedded in a hillside, and with 16 decades of erosion if finally came to surface? Cole Hill in Plymouth was used as burying ground in the early days of the colony, but after about 100 years of rains, bodies and bones starting sliding down the hill.
My wife’s father and grandfather both farmed the hills in Iowa and occasionally find bones & they would call the sheriff but it always ended up being Indian remains. I worked with her cousin on a house clean up job & he shared that both the grandfather and her dad were killed on the same tractor in the field where the remains were found. I remember the hair on my neck standing up when he told me the story.
Similar thing happened at an old Negro Cemetery that went back to before the Civil War, in Mississippi way back when I was a kid.
The old wooden coffins had long ago rotted away and they had not been buried very deep, 3 feet maybe, plus it was on a hillside that got soaked with rain and eventually gave way................