What happens at Seth and Thursday's condo that causes her to be hospitalized in the novel, The Wives?

After Thursday had dinner with Hannah, the dinner from which Hannah left while Thursday was in the restroom, Thursday returns home to her condo to discover that Seth is there even though it is not Thursday. Seth confronts Thursday, telling her that she is getting sick again. He lunges at her at one point. When she struggles against him, she slips and falls backward, hitting her head. Seth has her taken by ambulance to the hospital and admits her to the psychiatric ward again.