To be clear, journalistic assessments of the kind above refer to "IMPLIED volatility", ie the expected and priced level of future vol from options, that is effectively a measure of investor sentiment (as opposed to an observable economic reality).
This matters in so far as observations about the REALISED (as opposed to implied) volatility of stocks and how these might relate to other asset classes and the economy are based on a different (but obviously related) measure of the same concept...
So to answer the original question, there are two issues at work here.
The first and most obvious is the massive "left skew" in equities. They (long before options were liquidly traded) have tended to "grind up, and spike down". If you had to rationalise this behaviour economically, hark back to the Merton Capital model. Credit is the short put on the corporate's balance sheet; Equity is the long call. Anything that is "good" or "bad" for the corporate thus has non-linear "gamma" effects on stockholders and or versus bondholders. Given the capital cushion from the credit, the implicit "strike" on these options is out-of-the-money; you only start to see these effects once you start to stress-test the balance sheet.
The link in the FT oped to Fed policy is the other angle on this: the convexity of long-duration asset prices to changes in discount rates. Imagine a perpetual bond - it's the easiest thing in the world to price. Price = 1/Rate. The lower Rates go, the faster the change in Price per change in Rates will tend towards infinity. This is just a truism of asset valuation. That's the "convexity".
So the price of a perpetual bond is "Price = Coupon/K" (where K is the discount rate). Well, the original classic equity valuation is the so-called DDM (the dividend discount model) where "Price = Dividend/(K-G)" (where G is dividend growth). Assuming G>0, then stocks will be more converse, and thus more sensitive to discount rate changes, than perpetual bonds!
In the "good old days" (ie where textbooks live, in a world before 2008), it would be assumed that anything that lowered K thus would reflect slower growth that was also bad for G. So stock valuations would be (sort of, almost, mostly) invariant to interest rates. As it happens, I was an investment bank stockmarket strategist back then, and we used to have "what stocks win and lose when rates rise or fall" set up, almost as a zero-sum game back then.
The implicit but unspoken assertion in the FT oped, but it's "dogwhistle" if you know the code, is that K for bonds versus (K-G) for stocks (and even maybe the gold price) have all just become the same thing, courtesy of endless QE.
In which case, it would then be perfectly natural to assert that the volatility of the stockmarket was just as a leveraged expression of asset price volatility ('cos G>0), that was just a natural expression of interest rates ('cos of convexity 101).
I don't believe this narrative 100% myself; but it is (I think) the most internally consistent answer to the question you asked, as you asked it.
hope it helps,DEM