It appears she was run over by the car. That would be the most logical explanation. Another would be some sort of stress related issue like a stroke or heart attack.
we are so sorry this happened. Our thoughts are with you.
I can tell you that her death was not related to the vehicle at all she was not run over nor was she hit by the truck!! I believe it would have had to be some sort of health issue, people have said aneurysm or maybe blood clot would probably more so than a heart problem, as she had no previous symptoms and has been very healthy and active her whole life!!
I too am very sorry for your loss. Anytime we lose a pet, especially one that is apparently healthy and wheneve it happens suddenly, its certainly a shock.
Did you have a necropsy done on her? (An animal autopsy). That could have shed some light on the cause of her death.
Any other comments that we might make would be nothing more than guesses at this point in time. You mentioned that she was a healthy active dog...when was her last veterinary visit?