Remember the scene in the "Jerk" with Steve Martin clamoring to get away from "cans" when the sniper was firing at him...similar scenario...the key is to avoid park like areas.
He has NOT yet started firing at moving vehicles...but may do so in the future...
And that movie also provides a motive of sorts by virtue of one of my all-time favorite lines:
"Johnson, Navin R..... Typical random bastard!"