Not many options I can think off.
1. She loves Yuna or Minji and wants to get them apart.
2. She is jealous of their relationship.
3. She likes to mess with people.
4. Something happened in the past between Yuna and Hyera and she wants revenge? (Maybe Hyera loved someone back then but that person was in love with Yuna)
5. Something I didnt think off.