I really couldn't say why
Demi had a sudden rise in popularity.
My friend named her daughter
Demi seven years ago, and it was popular then, but I thought it had become less common since. I will say, though, that it's usually the lower-class, 'scally' types who call their daughters
Demi, and that Demis I have known tend to have siblings with names such as
Jodie, Lynzy (spelt something like this, never
Lindsay!) or
Shane.