From supply and demand -- if, say, 1000 people wanna buy something, and only a handful of people have that product, they (the sellers) will basically dictate what price they will sell it at.
Now, if these same 1000 people HAVE to buy something, and the people holding this something know this, basically they can charge whatever they want for it. In theory, whoever wants to buy these shares will have to pay exhorbitant prices if no one wants to sell -- so, let's say if no one is willing to sell it at, for example, 20 cents, maybe someone might be willing to do it for, say, 40 cents. Once all the shares at 40 cents are gone, it creates the same void, so maybe they would have to buy it at 50 cents. Rinse and repeat ad nauseum. Keep in mind, though, that this is in theory -- the actual results might be quite different, we'll only know if/when it happens.