Heard this from an NHK program last Wednesday covering issue about grave abandoned/forgotten by the family, mostly because modern day peoples no longer live with their older generation and thus lost much contact with even their closest ancestors.

One of the story from the show is that family grave where all family member go into the same grave is actually a practice more recently decreed by the emperor/shogun (perhaps Meiji, but I can't hear it clearly) so the living members can be more connected to their family and ancestors.
Previously Japanese peoples are buried in personal grave.