I don't quite know what to make of this. I thought it might be a trend due to people falling out of stable jobs and thus relying more on cash for payments, but

I don't quite know what to make of this. I thought it might be a trend due to people falling out of stable jobs and thus relying more on cash for payments, but the trend in hoarding large-denomination bills seems to refute this. The cash trend seems to be correlated with gold demand, but that too doesn't make a whole lot of sense if you're fearful of economic disaster - gold is hedge against inflation or currency collapse. Cash is only really a hedge against bank collapse, but bank deposits are generally backed by the government, so holding cash only protects you against temporary bank runs.