Explanation :
When the car accelerates, our body pushed backward against the seat. This effect is due to inertia.
When the car starts suddenly, our feet also comes in motion. But the rest body is still at rest.
Inertia is the property of a body which opposes its state of rest or state of motion.
In this case, the inertia of the passenger resists the motion of the car. So, he pushed backwards as the car started.
