I suspect that part of the volatility, besides the changing fundamentals and behavioral factors is also attributed to portfolio optimization process. I haven't done any research on this subject on my own, but having studied portfolio management, it seams like a logical consequence. My rationale behind this is that continual balancing of the portfolios (especially the tracking portfolios) is driving trading volumes and with it the volatility.
Is there any research on the influence of the modern portfolio theory on the increased volatility and the instability of the volatility in the past decades?