If the vehicle is not securely parked sufficiently, it can roll away in an uncontrolled way even at a slight downhill gradient.
Flammable material such as leaves, grass or twigs may ignite if they come into contact with hot parts of the exhaust system or exhaust gas flow.
Open doors, thereby endangering other persons or road users.
Get out and be struck by oncoming traffic.
Operate vehicle equipment and become trapped, for example.
Releasing the parking brake.
Changing the transmission position.
Starting the vehicle.