It is a sad thing for someone so young to pass away so suddenly.
I would attribute her death to the stress of her life in general.
I have not really heard anything thus far, as to how she died, but if it is due to natural causes, stress can start doing some pretty harsh things to your body, and mind and basically start shutting you down.
Her wieght loss, was so dramatically fast, I would wonder if that was a defining factor. I would assume that losing that much weight, in such a short period of time, would not be a healthy thing.
Top that off with a birth, a death, and constantly being in the spotlight, and not always in a positive way.
That I think would put enough stress on a person, to make their body work against them.