Buying any asset pushes the price up and selling pushes the price down. It's a matter of sensitivity. The Chinese government could probably push down the price of USD a few percent by announcing a policy change today, but one guy could potentially push Bitcoin prices down significantly with a keystroke.
Interestingly enough, that's how Société Générale lost huge amounts of money following the discovery of Jerôme Kerviel's misbehavior. They recklessly closed their positions, shooting themselves in the foot in the process.
