Might another vehicle have collided with the tailgate while your car was parked?
A small rock kicked up by a tire? One local owner, same thing, car parked in private apartment complex lot, broken tailgate glass overnight. Security video showed another car pulling out right when the tailgate shattered, and they found a thumbnail sized rock in the glass fragments on the pavement. Neighbor had the front glass door on their house shattered by a lawn mower kicking up a rock from 30 feet away.left my i3 in the car park at 7:30 am went to my office, returned to my car at 5:00 pm and the rear tailgate glass was shattered, its a private car park single bays the car did not move all day !