It is a rare glitch that occurs in the game. When the player exits a vehicle, shoots another vehicle so it flees away and then shoots his own vehicle, his vehicle may or may not move by its own like it is fleeing. The reason behind this could be a simple A.I. glitch where the game thinks that the vehicle has it's driver inside so it flees. Another reason could be that as soon as the player exited the vehicle, a car thief had entered the vehicle and fled as the bullets hit the car. Still, this glitch is yet to be confirmed.