In the beginning of chapter one of The Liar's Club, why does Mary Karr begin with that incident?

Mary Karr's poignant and disturbing memoir The Liars' Clubstarts with an incident that occurred when she was seven. On that night, Karr's mother set a fire to burn the children's possessions in the backyard when Karr's father was at work, and Karr's family doctor investigates her to see if she's been physically harmed. The incident is important in the circular narrative Karr writes about her troubled childhood because it marks the beginning of her...

Mary Karr's poignant and disturbing memoir The Liars' Club starts with an incident that occurred when she was seven. On that night, Karr's mother set a fire to burn the children's possessions in the backyard when Karr's father was at work, and Karr's family doctor investigates her to see if she's been physically harmed. The incident is important in the circular narrative Karr writes about her troubled childhood because it marks the beginning of her constant vigilance to watch for events in her house that she calls "Not Right" (page 9).

Karr writes poetically of that night:

"On the night the sheriff came to our house and Mother was adjudged more or less permanently Nervous, I didn't yet understand the word. I had only a vague tight panic in the pit of my stomach, the one you get when your parents are nowhere in sight and probably don't even know who has a hold of you or where you'll wind up spending the night" (page 7).

Karr's mother is hospitalized for being psychiatrically unsound, and her father is not around. Karr is left on her own with her sister, and it's clear that no one is really taking care of her. 

Karr knows from that point on that her life is going to be chaotic and that her neighbors know that her family cannot take care of her. She writes, "I did know from that night forward that things in my house were Not Right" (page 9). She translates this sense of her house not being right into her sense that she isn't right. "The fact that my house was Not Right metastasized into the notion that I myself was somehow Night Right, or that my survival in that world depended on my constant vigilance against various forms of Non-Rightness" (page 10). Her use of the word "metastasized" is poetically apt, as her town has a chemical plant and oil refinery that cause many people to have cancer. For Karr, the cancer is the dysfunction in her family, and the incident that begins the book establishes her family's broken dynamic that she will explore in the rest of the book.