snip:
As money, it is on the other side of all asset prices. In other words stocks, bonds and property prices can be expected to rise measured in gold when the gold price falls and vice-versa. This relationship is often muddled by other factors, the most obvious one being changing levels of confidence in paper currencies against which gold is normally priced. However, with bond yields today at record lows and equities at record highs this relationship is apparent today.
Another way to describe this relationship is in terms of risk. Banks which dominate asset markets become complacent about risk because they are greedy for profit. This leads to banks competing with one another until they end up ignoring risk entirely. It happened very obviously with the American banking crisis six years ago until house prices suddenly collapsed, threatening to take the whole financial system down. In common with all financial bubbles everyone ignored risk. History provides many other examples.
Therefore, gold is unlike other assets because a rising gold price reflects an increasing perception of general financial risk, ensuring downward pressure on other financial asset prices. So while the big banks are making easy money ignoring risks in equity and bond markets, they will not want their party spoiled by warning signs from a rising gold price.
http://www.zerohedge.com/news/2014-09-14/why-rigging-gold-market-matters