The meditator in this photograph is sitting too high (notice the exaggerated height of his seat). What results from this is that the pelvis tilts forwards. In order to avoid falling flat on his face, the meditator leans back, causing an exaggerated hollowing in the lower back.
There should, of course, be a normal, gentle hollowing in the lumbar region (it's called the lordotic curve) but exaggerating this will cause a painful "pinching" in the lower back. Over-arching can also occur when the angle of the seat is too steep (this also throws the weight forwards, so that the meditator has to lean back, causing excessive hollowing). And over-arching can also be a simple postural habit.
The solution for over-arching is generally to adjust the angle of the seat or (and this is more common) find a lower seat, or to make an adjustment to the angle of the pelvis.
