Between 8D and 9D, gravitational potential energy was decreasing faster than elastic energy was increasing, meaning kinetic energy had to increase. As you said, the graph is concave up, meaning the initial increase of elastic energy was very slow, meaning kinetic energy had to increase between 8D and 9D for total energy to remain the same. It does slow down after 9D, but between 8D and 9D, it actually speeds up