Coincidentally, it was also an excellent numbers week for 'Light Plane Maintenance' (up 741% in Magazines) and the quantity of people worried about the government reading private emails.
In the case of Nineteen Eighty-Four, There's been no Christmas Number One campaign, no Facebook event championing the cause, no rise in Google searches for the book or its author (See below). This was just one of those things. A natural swarm towards Orwell's chilling vision of the future; an organic response to recent developments.
Last week's Channel 4 documentary The Human Swarm is nice background viewing on this topic; how purchases are dictated by external factors like the weather or the news cycle. It's genuinely interesting stuff and well worth your time if you're interested in this sort of thing.