The timing you mention fits in to the timing of the referendum and it's result giving further indication, if any was required, that people are uncertain of the future.
So why were sales so low from 2005 to 2014? maybe that was because we were inextricably linked to the EU, and sales shot up for a while when people saw the possibility of an escape?
I'm not claiming that as a link, just showing that extreme generalisations can be made, on very little evidence, by either side in the discussion.