When we pressed again the seat from back side
So we can say initially we are moving with car at same speed
now when driver press the gas pedal we are pressed against the seat from backside
So it means the car moves ahead after pressing the pedal with larger speed and due to inertia we are still at our initial speed
Now the car will press us from back side as car want to go ahead with larger speed
so here we can say car is accelerating towards forward direction
so its direction of acceleration is in the direction of initial speed of the car