> Bond mutual and exchange-traded funds now own 17% of all corporate bonds, up from 9% in 2008, according to the ICI. In periods of market stress, more-concentrated mutual-fund ownership tends to mean larger price drops, the IMF said last year.
It used to be the case that stocks, bonds and other commodities like gold has inverse correlations. Or in other terms, when stocks were down, bonds and gold were up....
This lead to the Efficient Frontier from Markowitz where you would choose a risk level you were comfortable with and then build a portfolio of "uncorrelated assets" to get you optimum return for your risk level.
http://www.investopedia.com/terms/e/efficientfrontier.asp
The issue now a days is that this no longer makes much sense as when things go wrong everything is correlated almost to a degree of 1. Or in other terms, when the shit hits the fan, everything (Gold, stocks and bonds) all go down together.
TL/DR the old advice about investing in both bonds, stocks and gold for diversivication of risk is at best much less pronounced than it used to be and at worst just bad advice as they are now positively correlated.