After Uncle Frank's suicide David's mother wished to move away. They'd been snubbed by her husband's family at the funeral, and the accusations made about Frank before he killed himself would forever separate them from the rest of the family. David's mother instigated the move, but his father in no way tried to thwart her desire to start over again somewhere else.
David's mother didn't want to live with lies. After Frank's death, nothing that was said was true. She could live with the deaths and the suicide because her faith in God could shelter her from those things. What she couldn't live with, and the reason they had to leave Bentrock were the lies she refused to acknowledge as truth.