After Luna had written to her about how she wouldn't be able to make it to Ponyville for Nightmare Night, Twilight decided to organize a small camping trip to the outskirts of the Everfree Forest with her friends. She'd initially planned to have Trixie come along and entertain them with a small show, but unfortunately the unicorn had declined the offer, saying that she had something else to do.
So, since it was, after all, Nightmare Night, Twilight decided that the best thing to do to pass the time would be to tell scary stories to each other around the fire. She was not expecting her friends to each have such a diverse concept of what "scary" means.