Until President Nixon abolished the Gold Standard in 1971, central banks had full control of the bullion market as the value of the Dollar was tied to the gold price. It was illegal for a U.S citizen to own gold so all the gold in the markets was held in the bank's vaults. This system ensured a steady but slow economic growth since governments could just create more money to boost the economy.
After the abolishment of the Gold Standard, the price of gold rose from $43.35/oz up to $850/oz because everyone wanted to invest in gold. People didn't trust the paper currencies as they weren't backed by any physical asset. This didn't please central banks so the U.S with the help of the IMF tried to limit gold sales through auctions. This didn't work out because in reality the banks wanted to keep the yellow metal so the limitations were withdrawn.
After that the banks tried another tactic, which worked out well up until 1999. They lent their gold to gold miners to finance their operations, which created a massive over supply of gold and the price fell as low as $275/oz. This technically allowed central banks to keep their gold reserves since miners would pay them back with gold from the mines.
At the same time central banks threatened that they would sell all their bullion over time, which ensured the Dollar's position as the only reserve asset as it was the only currency to purchase oil with.
After Gordon Brown in all his wisdom decided to sell half of UK's bullion reserves in 1999, the IMF decided to limit annual gold sales to 403.3 metric tons. This removed the fear that central banks would sell all their gold and the gold price started a new bull run.
Central banks still had some form of control over the gold price after the IMF announcement until last year. For the last 20 years European central banks have been selling their bullion reserves and that way controlling the gold floating into markets.
Last year gold sales from the central banks stopped and they have started to buy gold bullion. When the banks stopped controlling the supply of gold bullion, they also gave up the control of the price.
As the old Western nations are paying the consequences of their loose monetary policy, the emerging economies from the East are enjoying healthy GDP growth figures. Such large nations as Russia, India and China have been buying more gold than the miners can supply, which has pushed the gold price up to the current levels.
Western central banks are facing a dilemma with their falling currencies and the rising gold price. If they start to purchase large amounts of gold, they would be admitting that they don't believe in the current monetary system. This would cause panic and would destroy even the smallest hope of recovery.