The Libor (London interbank offered [loan] rate, which is the rate at which banks lend to one another, and which indicates how much banks trust one another's financial health) was manipulated. The gold market is smaller than the amount of gold ever mined. Much of the gold ever mined remains permanently unavailable to the market. Therefore it would be easier to manipulate the gold price than it would be to manipulate interbank lending rates, and the Libor.
Governments and central banks depress the gold price when they wish to support their currency's exchange rate.
Courtesy of, and with thanks to, Mineweb's "LIBOR scandal brings gold price manipulation once more to the fore". Also see The Telegraph's "The price of gold has been manipulated. This is more scandalous than Libor".