How is it that a stable person (for their whole life) suddenly up and commit suicide?

It can happen. It is almost impossible to know the soul of another person unless they choose to share it. What we see and hear from others is just a small portion of who they are, a video clip and not the whole movie. It is tragic & occurs seemingly without warning. The other half of a story like this is that life changes and even the strongest of us can fall into suicidal depression. Be vigilant for signs.
Suicide. Hopelessness, major life changes, antidepressants with activating effects, impulsivity, no support system, accidental.