Mufasa is a trained animal, but the trainers were still a little uneasy keeping the lion out in the open like this. They were careful to keep everyone standing, especially the children. From what they told us, lions perceive any creature smaller than themselves as prey, and if you're sitting or lying down, they assume that you're wounded and will pounce. So the animal might have been trained, but it still has wild and dangerous instincts.