How did they decide to explain what happened to Frank?.

In the Epilogue, we learn that David's family leaves Brentrock. He says that the decision to move was made shortly after Frank's suicide, and that the overriding reason for the move is that his mother has no way to deal with the web of lies created to cover up Frank's suicide. The story circulates that Frank had been helping Wesley build new shelves in the basement and that Frank had fallen from a ladder, bumping his head and causing his death. The funeral director, Clarence Undset, was bound to know the truth but David says everyone seems confident that Clarence will never reveal his knowledge.