they would say the reason if it's something that she or jyp wouldn't get backlash for. imo, it was probably
1. dating. the jyp dating ban is the first 3yrs since the debut, so this was probably a problem.
2. relationship with the company. it could have been a fight with a staff member or maybe she showed strong dislike of the company.
3. a possible scandal. jyp might have found something in her past that could have been a problem, similar to the Garam scandal.
4. she just didn't like the idol life. this seems the most likely, as the kpop industry is putting more pressure on idols to be perfect