Haruki hears about Kazusa's moving from her mother, who she hadn't been on good terms with.  He feels betrayed and angrily confronts her.  But his words force open the door to Kazusa's heart, which she had desperately tried to keep closed so that the three of them could stay together.
How did you know?
Woher weißt du das?
¿Cómo lo sabes?
¿Cómo lo sabes?
Comment es-tu au courant ?
Como você sabia?