-foreword; the next two chapters are purely for story development. They are not necessary.-
Before the first changeling queen, Queen Iris, stepped down from the throne, she made sure that every changeling queen, and every pony in Equestria would remember her reign of terror. The thousand-year war with princesses Luna and Celestia would not leave them the ultimate victors.
She put a curse on the land, that any future element of harmony that disrupted the plans of any future queen would be sentenced to a life of discrimination. The princess of the changelings at the time did not share her mother's desire to rule, so the curse lay forgotten as time past, and by the time the next ruler came, Queen Chrysalis, the curse was forgotten entirely. But it still remained.
Queen Iris' wish to take revenge as ultimately granted, when Twilight and the other elements disrupted the wedding, letting the curse arise upon them. Which one? Who knows? Oh wait, but I do. That would ruin the story if I told you, wouldn't it? I suppose you will have to wait then. But fear not, the answer is not far. Let the story begin.