What traumatic event occurred while Mary and her family were driving from Leechfied to Lubbock in the hopes of outrunning the storm?

While driving away from Leechfield to Lubbock, Charlie began her descent into madness. She was singing softly under breath while driving erratically on the wet road. When she reached the chorus of the song she was singing, "Mack the Knife", she threw the car into a 360-degree spin until it was out of control. The car crashed through a pedestrian walkway and slammed into a railing. Mary vomited on herself and began crying in the backseat, but Charlie seemed unfazed. She righted the car and asked in a cheery voice if everyone was okay.