It may be central banks, but it could also be any institution or individual with a few billion dollars. The cryptocurrency market cap is so tiny at just $70 billion ( that's less than 1/10 of Apple's market cap) that the price action of entire thing is relatively easy to manipulate for someone who understands price action and has the financial resources. However, unlike Gold and Silver which have the paper derivatives that allow them to be manipulated for years on end, cryptocurrencies can bounce back quicker