There were some indications that she left the team on bad terms last season, since she was spotted out in public in March (in her hometown Pohang) even though the season wasn't over yet and Hyundai later clarified that she left training without permission and without contacting anyone.
I don't think there was any major discord between the player and the team. Hyundai wanted her to come back and tried to persuade her to return but couldn't. That is why they didn't announce her departure from the team straight away. We can only speculate as to why she decided to quit playing volleyball. Some of the articles that came out today mention that she receives criticism and malicious comments online.