I also wondered why anyone would plant a tree right on top of a grave.

there are cemeteries that offer that it's a fashion like a sidecar hearse, my choice would be our local funeral directors called "Ernest Cox". And FWIW anyone can set up a cemetery apparently, but you have to inform the council (which one wasn't specified, parish/county/unitary authority)

And the advice about planting a tree over a coffin was broadcast on "Gardners Question Time" BBC Sundays afternoon - my kind of expert to rely on reasons for being less that helpful to the tree were not given or weren't remembered.