that was probably jiwon... apparently she was the only one who wanted to leave the group at first but she didn't want to leave alone so she pressured jiae into leaving with her
whatever happened seems to be fine now since Jiwon still spends time with the other 3 original members and even has couple rings with eunjung
anyway here is the article about that:
http://www.allkpop.com/2011/02/hyun-...new-girl-group