I think there's two possibilities here:
1. Mistreatment = they had enough, probably didn't get paid well for their success, new contract was a joke so they all left together and disbanded.
2. They'd had enough of being idols = we all know they were worked to the bone before debut, 6 years in they might have actually made some good money to the point they thought this life is no longer for me.
I think it may be 1. as if you wanted to relax a bit more you could simply dial down the comebacks but stay with the company as you'd still be selling 100k albums and getting CFs.
What I'd like to know is were HYBE informed of this, did they join any discussions and try to intervene or did they leave it all to Source Media? If I'd bought Source with GFriend as the main earner I'd be totally involved in something as critical as contract renewal surely...