Could the ground have eroded over time and left those graves up above the ground? It kind of looks like that could be it, especially with the broken ones. I was tempted to peek into a broken once myself, but in the end I couldn't make myself do it.
The graves are above ground because the water table is so high that it bascially pushes up buried things in time. If they're going to wind up on the surface, may as well start them out there.
There are several old cemeteries by us on Long Island. Minute men are buried there. I find them fascinating. There are several above ground crypts that are fenced off with beautiful fencing in between other graves so I dont think they had risen from water. The tree roots did leave them lop sided. There are clubs that do rubbings on tomb stones and there are books about where civil war, famous people are buried. There is a burial ground in the middle of our Home Depot parking lot that is fenced off. There are at least a dozen family cemeteries in neighboring yards in a one mile radius. I saw a show about a person finding tomb stones in their yard and it was just that they were replaced at some point and dumped in the persons yard. still cool thing to find