It think it has a wider definition than just 'history' because it means 'the story behind the story' which can be current and ongoing iyswim.
We are all more aware of 'backstories' to items in the media now because of the internet and wider access to information. You can research the backstory to something or someone that interests you. I wonder if this is why it's being used so often now?
It seems an appropriate word for mumsnet, anyway! I love a good back story.