It sounds like someone made a bomb built into the tire, which was triggered to go off when the doctor moved the tire. It's some kind of homemade bomb. Whoever did it probably intended to kill the doctor and didn't expect him to survive. They either had the bomb rigged up to the car engine (when turned on) or tamped it in that direction..
Mornin' MeoW333
That leads me to believe this might be a more sophisticated perp than your average joe, what do you think?