Discord is balance between good and bad. Light and Dark. So whenever there's an imbalance, he must counteract that imbalance. What happens when the imbalance is a pony? And... he gives them a choice? Either he stays, or they both go to stone.
When the Queen of the Changlings takes her revenge on the ponies, will Twilight be able to stop her? Or, more importantly, will Twilight be able to stop herself?