It's easy to predict the motion of a single pendulum. The motion of this large pendulum is more complex because it has smaller pendulums attached to it. Each of these pendulums influences the motions of the others, making the overall motion chaotic and unpredictable.
If you could start the pendulums swinging exactly the same way each time, they would always do exactly the same thing. But tiny differences in the way you start them can add up to drastic changes in their later motions. This extreme sensitivity to starting conditions is a characteristic of chaotic systems.
