It’s in a final flashback sequence that we discover the source of Brynn’s isolation in her town. This is due to what happened between her and Maude, a former childhood friend and the mysterious subject of the letters she obsessively writes throughout the film. While playing in the woods when they were children, Brynn and Maude had an argument. Like most of the movie this plays out in silence, so we don’t know exactly what was said. What we do know is that Maude shoved Brynn over, and Brynn reacted by picking up a rock and hitting Maude over the head with it. Maude, presumably, died from the blow. This is the reason Brynn’s the subject of stares about town, and why Maude’s mother spits in her face at the police station.