A closed cubical box is completely filled with water and is accelerated horizontally towards right with an acceleration a. The resultant normal force by the water on the top of the box

As the box is accelerated in the right, the water experiences a pseudo force on the left so as to maintain its inertia. This causes the water to shift to left. As more water is present on the left side than the right side, the resultant force also shifts to the left.