The short answer is "depression", but that's a trite and superficial pretext of an explanation.
I will always believe that it was because the sheer depth and immensity of the evil and depravity in which she immersed herself in order to do her research simply overwhelmed her...
I agree. I also think that her acute sensitivity to injustice throughout her life, even as a child, magnified the despair she felt for persecuted people.
Writers in Davis, CA told me she suffered from bipolarism.