She didn't exactly die of a "heart attack." I believe her cause of death was listed as cardiac arrest that resulted from blunt force trauma to her chest.
I don't think the exact cause of death is the main point here. If John S Mosby is correct, the guy wasn't deliberately hitting a person, he was hitting the back of a vehicle with no knowledge that a person was two cars further down standing in front of a vehicle, and would be caught in the cascade collision.
Is this part correct, or does anyone know for sure? I had read they recovered her blood from his bumper, but that might have been wrong.