Question

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A curious and righteous Christian friend asked me the following today. She heard that in Jewish tradition, when a casket touches the ground it cannot be raised up again but must be lowered into the ground. I am not sure why a casket placed on top of or on the grave might need to be raised again, in my experience, but I wonder if there is minhag here, or a halachic requirement? Can any one give me an historical perspective? Thank you in advance for any ideas.
David