My wife did the same thing, packed her bags and left her daughter and the daughter we were adopting. It is pretty messed up. Did not tell a person she was going to leave, we had no fight nothing. In the end her "knight" turned out to be a drunk bum, and she is now on her 4th guy since she left.
ISSUES.
Me 33 WOW 31 Married 12 years 4 YOD 1 YOD(adoption process) Bomb & moved out 4/01/09 Divorced 12/17/09