Let me explain. Rosa Gale had a fear that Lightning Dust would abandon her. So Rosa hid her real talent which is aerial ballet and tried to train to become a fast flier, through excessive training. The reault of this was disastourous. Her flying skils and physical health were declining. Her father and mother noticed and decided to follow her. It was there in a clearing in the forest that they learn that she already gotten her cutie mark and her real talent. While Lightning Dust was mad at , she was heartbroken because she accidentally put her daughter that through this and promise to support her.
 
Fun fact: it took me a long time to create the proper expression for Lightning dust.