The shockwave from an undersea earthquake causes 'ripples' in the ocean which are very large swells. When the bottom of the swell touches land, it must build up higher to maintain it's size. Therefore when it approaches the beach, water is drawn away from the beach to compensate for the ocean depth getting shallower under the tsunami swell. More water is drawn from in front of the swell than behind due to the movement of the swell.