She was once friends with Starlight as foals and had issues coming out of her shell. When she finally had the confidence to throw a surprise party for her best friend, Starlight never showed up. It was on the day that Starlight had been sent away to Ponyville but that didn't help ease Trixie's pain.
She was so devastated by this that she became a recluse and refused to talk to anyone until Starlight visited her old home again. After making ammends, Trixie is much happier and is more social.