The common denominator, in my opinion, is the widespread use of wrong equity valuation methods. While much of the active managers believed the stock market was overvalued, it wasn't. Measures such as P/E and Shiller's CAPE failed miserably. Consequently, dumb index trackers easily beat the 'smart money'.
Is there something better than CAPE? Yes, of course, it is. You'll learn about it in one of my posts in the near future.