I'll guess this has been asked and answered more than a few times, but I'll give it a quick go:
Bitcoins go up and down in price in response to changes in Supply and Demand.
That is it. Since production of Bitcoins happens at a fixed rate (over the long term), the Supply side of that equation is always worked into the price. So essentially Bitcoins respond to changes in Demand.
There is no "man behind the curtain", just simple economics 101. Think of anything that could increase or decrease demand for Bitcoins and you will have one of the many drivers of bitcoin pricing.