I doubt that a woman who has trained and looked forward to space travel as much as these folks do, would choose to get pregnant at just the wrong time.
So either an accidental pregnancy - or more likely, some sort of behavioral problem, with an incident that made it impossible to ignore, and impossible to allow into a sealed space station for five months.
Perhaps. . . but I still think pregnancy is the reason.
I wondered that, but she seems a bit old to be expecting.