Mrs. Piganti died of natural causes, probably from a sudden illness because she died before her husband.
Although Lorraine is convinced that Mr. Pignati killed his wife and buried her body in the basement, she has the overactive imagination of a teenager. When John rifles through Mr. Pignati’s things and finds the funeral bill, he realizes that she is dead. Lorraine’s first through is that Mr. Pignati killed her because something is off about him. His wife is certainly not away on vacation.
I thought it was very sad. I mean, the cute little girl in the ruffled dress had already grown up, gotten married, lived her life, and was underground somewhere. And Mr. Pignati wasn’t able to admit it. (Ch. 8)
A sudden illness would make sense for Mrs. Pignati’s death, because he never came to terms with it. She might have also died from an accident. He never accepts her death, because accepting it is too painful. It is easier for him to go on pretending, in simple ways, that she is alive. He says she isn’t home., and talks about her in the present tense, saying she “loves delicacies” as if she is going to continue using her credit card. These self-delusions are the result of the fact that he is not ready to let go of her.
At first Lorraine thinks she just left him, until John finds the funeral notice. The fact that there are no other signs of her death speaks to a sudden death.
That wife of his—Mr. Pignati’s wife—looked like she didn’t take anything with her to California. All those clothes in the closet. (Ch. 7)
Often after a sudden death, the remaining loved one with leave everything as it was, as a shrine. This seems to be what Mr. Pignati did. He was unable to face what happened. He told the kids she went to California, but people close to him certainly knew the truth.
The cause of Mrs. Pignati’s death probably does not matter much to him. He is grieving and clearly weakened as a result. He is lonely, and sad. The fact that he opens up his life to these two teenagers shows how desperate for attention he is, but also how vulnerable he is. It is a move he will regret, unfortunately. Although they might bring some company, they will also unintentionally bring him to his destruction.