Hank Green has a really good explanation about this in a video on the Vlogbrothers youtube, starting at about 7:33 - https://youtu.be/hIGtTImeYU4?t=452

The short answer is, since the reactor was cooled with regular instead of heavy water (hence, cheaper), pulling the control rods out didn't speed the reaction up as much as it would if they were to use the (more expensive) heavy water, as regular water absorbs more neutrons. So, instead of just "removing" control rods, the reactor pulled in graphite rods behind the control rods. (Graphite being a substance that would "speed up" the reaction, thus counteracting the slowing of the water).