When he was found, was there any sign of forced entry on the car? If there isn't, could it be that he might've left the car by his own will? These are my speculations:
1. Something happened and he left the car with the key. Outside the vehicle, he got robbed/attacked. I don't have any evidence to suggest this, considering I don't know the exact location. But this might be wrong, considering he was in the army, so I doubt he was an easy target.
2. Suicide, did he have any mental health problems? It could've been an unplanned one, but no evidence backed this up.
3. Some other possible situations: Chased by another car, running away, etc.
Again, this wasn't backed up by any evidence and only my speculation on what possibly had happened. Please do inform me if some of these have been discussed before.