Many exchangers manipulate the market when they dump and sell their coins. Exchangers who involve in Pump and dump help in promoting a stock they hold and selling once the stock price has risen following the surge in interest as a result of the endorsement. This in turn causes a misleading appearance and could affect the prices
They can manipulate it anytime they want. It is somehow their power to create a strong and enough volume to their exchanges. I have know some exchanges that indicates that the coin that I am trading with has a volume but when I put my order something weird always happen, you may observe it in the trading history. There are many fake volume i many exchanges now. There are few that it is real now.