Pretty sure there's no confirmed answer, but a few people have theories on it. A lot of the theories are around either Jacques or Max killing her, but they are just theories so it won't look stupid for you to do whatever you want with the story. Though for the record, even if there was a confirmed obvious answer, you can still do what you want with the story, lots of people go against what's 'canon' for their sims. It's always cool to see a new take on something, even if it isn't the story the developers intended. :)
