Bitcoin markets aren't regulated and exchanges can potentially just manipulate the volumes like what the willy bot in mt gox used to do. The price is determined by how much the sellers are willing to sell at and how much the buyers are willing to buy at.
I think that was the case in the history but I am not sure how much is this going on nowadays! OK Chinese volume manipulations are there but they are not manipulating and raising price artificially like Willy the bot did.
You do understand that this is very challenging today when you have about 10 quality exchanges. 2 years ago we pretty much only had Mt gox and stuff like this was much easier to pull off!