It was never the goal of the nightmares to make the dreamers fear them, and they always were doing their best to stay away from dreams.
But recently, something has been hunting and killing the nightmares. It has much power and glows a pale, sickly, light blue color, and therefore was nicknamed 'the presence' by the nightmares. One could sense and see it from far away.
This is what pushed the nightmares to become active for the first time, to try and intentionally hurt another being, instead of just scaring them.