The phenomenon of RORO – risk on, risk off – is nicely illustrated in two graphs from HSBC, via alphaville:
What you are seeing is two maps of correlations between various assets, in 2005 and 2012. Dark red means the two assets are strongly positively correlated, dark blue means they are strongly negatively correlated, and turquoise, green and yellow means no real correlation either way.
In 2005, most assets were roughly uncorrelated. Some, like the NASDAQ, S&P500 and Dow Jones, moved in tandem, as did the four key European markets, and the key sovereign and investment-grade bonds. But for the most part, different assets gained and lost value in an uncorrelated manner.
Come 2012, and everything changed. In the top left are all the assets which get stronger in the good times – mostly indexes like the FTSE, but also a few currencies and copper. In the bottom right, there are the assets used to hedge bets when times are rough: the sovereign bonds, the Yen, and right down at the bottom, the US Dollar.
The former class are the risk-on assets; those investors buy when they want to take on risk to make money. The latter are the risk-off assets; those which they buy to get themselves some stability.
The simplified reason for the change is the bimodal nature of responses to crises. When things go wrong, one of two things happen: Governments step in and save the day, or they don't. Quantitative easing is one example of this, but so are bank bailouts, expansions of the "firewall", and so on. If they happen, every risk-on asset rises; if they don't, everything falls.
For those interested, a deeper examination of what RORO means for markets is given by Bryce Elder over at the FT, but the overall problem with the phenomenon is that it reduces trading to a bet on up or down. As a result, traders hate it. As Elder writes, instead of being able to do their job well, by focusing on the fundamentals of each asset they buy (asking questions like "is copper going to be in demand because of growing infrastructure demands"), "each day’s profit or loss is determined to a large degree by results of a sovereign bond auction or comments by a central banker".
Until the crisis is over, though, RORO is sticking around, so investors had better learn to live with it.