When Zach was having dinner with bree and Bree told him something about her secret and Zack can't help release something about his bad memory. And he told bree her mother committed suicide because of something he did

Zach is actually very poor boy, he had so many bad memories when he was a child. And even after he grew up, he didn't have enough knowledge of himself.  On the contrary, he has so many things to blame on himself.  He may believe he had done something very bad that caused his mother to kill herself

When the accident happened under the witness of Dana, the whole family just did not talk about this topic, and what happened to his mother. Anyway, that's too bad for a young soul to have this great burden.