Not "everyone agreeing". If *most people* (most capital) agree, you are forced to agree, or you will lose money because price will move in that direction since most people agree.
Let's look at all the scenarios.
If you think BitUSD is worth $0.9, you will short BitUSD when it is $0.95 expect to be able to cover at a lower price. But if the price goes up to $1 and stays there you are forced to cover and take a loss. (You are incentivized to eat your loss because if it goes up too far there is a margin call).
If you think BitUSD is worth $1.1, you will buy when it is $1.05 expecting to be able to make a profit at a higher price. But if the price falls to $1 you can only get out at a loss.
On the flip side.
If you think BitUSD is worth $1, you will buy BitUSD when it is $0.95, pushing the price up a tiny bit. When it finally reaches $1 you can sell it for a profit.
If you think BitUSD is worth $1, you will short BitUSD when it is $1.05, pushing the price down a tiny bit. When it finally reaches $1 you can cover and sell your BTS for a profit.
So, assuming *most* other people agree, the price will move towards $1, hence if you agree you will gain money and if you disagree you will lose money.
So you will either lose all your money or be converted to a believer.
So it is a self-reinforcing belief that BitUSD should be worth one USD.