Firstly, if the center of the pelvis and knee is situated ahead of the center of the ankle (See the left image above), the arch of your feet are prone to collapse. As shown in the above video(05:15), bring your pelvis forward in standing and you will feel your foot arch go lower.

Secondly, if you look at the picture on the left (above), you can see the woman's pelvis is pushed to the right. In this case, the arch of her left foot is likely to go flat.

You can test it yourself. Stand up now and push your pelvis to the right. Then, the arch of your left foot will go down and the arch of your right foot will go up.

However, the arch of the right foot is also likely to be lowered soon because more weight is placed on top of the right foot.

The last one is when the pelvis rotates to one side, as shown in the last pictures. If you look at the left picture (above), the man's pelvis is rotated to the right as the left pelvis goes forward while the right side goes backwards.

If you stand up now and turn your pelvis to the right, the arch of your left foot will be lowered and the arch of your right foot will be raised. However, your right foot doesn't stay elevated. When the pelvis rotates to the right, the risk of right ankle sprain increases, so the right foot tends to firmly attach the inside of the foot to the ground to prevent it from rolling outwards. Depending on the degree of the compensation, the arch may go lower than the left foot.

In summary, flat feet can be a result of postural imbalance, uneven weight distribution and following compensation.