I think what Didion is saying in the excerpt from The White Album is that as journalists, and, in fact, as a society, we look for the reasoning behind actions. Nothing is as it seems at face value. For example, with the Charleston shooting, some news outlets (Fox News) tried saying the motivation behind the shooting was an attack on Christianity, despite the quote from Dylann Roof that he was "there to shoot black people." Different people and different outlets can find different motivations for actions which lead to tragedy. Even if they are outlandish and all evidence proves them false. I think that what she is saying is that we all have our own interpretations on motive which can be skewed by our own loyalties which can blind us from the truth.