Never Give a Trucker an Even Break - S1-E9
Continuity mistake: There is a scene where the female trucker is driving her truck and she intentionally hits a brown car. The problem is that she had already passed by this brown car a few seconds ago when David was getting into her truck through the window.
Answer: An illness, although it's never stated what kind of illness. David blames his dad for her death because his father never took her to the hospital. So in all likelihood, her illness was never diagnosed anyways and David might not even have known what it was.
Bishop73