why does this happen?

Someone on one of the message boards I post on passed away. She was a mother to 3 young children. It was sudden and unexpected, and no one knows why yet. Why does this happen? She had a 4 year old, 2 year old, and a baby around 6 months old. A BABY. And now she's gone. Luckily the children have a large extended family, and of course still their father, but they don't have a mommy anymore.