A falling human stops short when it hits the surface of the water, but its organs keep traveling for a fraction of a second longer, until they hit the wall of the body cavity, which by that point has started to rebound. The aorta often ruptures because part of it is fixed to the body cavity—and thus stops at the same time—while the other part, the part closest to the heart, hangs free and stops slightly later; the two parts wind up traveling in opposite directions and the resultant shear forces cause the vessel to snap.
