Answer:
Due to inertia.
Step-by-step explanation:
As per law of inertia, a body will remain in its state of motion until an external force, unbalanced in nature, is applied on it. When the car is at rest, the person sitting in it will also be at rest. When the car suddenly moves, the body resist the motion due to inertia and it stays in initial position while the car moves. Due to this the body will be pushed back.