What happened during the Christmas evening drive from Camp David to a campaign fundraiser? How did the car crash? Was that a tree or some kind of terrorist attack? And how did she (First Lady Margaret) die?
The crash was caused by part of a tree falling on the car. A few moments before the crash we see the camera focus on the part of the tree that is breaking. This caused the car to skid off on the ice and through the side of a bridge, it was the fall from the bridge that killed the First Lady. Quote from wikipedia: