You buy something if you think the price will go up, but this also makes the price go up. If a whole bunch of people think a commodity will go up, and start buying up, this will drive the price up a lot. A commodity does not have to change it value, or for there to be an increase in demand for the price to go up, all it needs is some speculation that it will go up, and a whole bunch of people looking to profit.
I still don't get it. If people are buying and the value is not there, they will loose money, right? Speculators can also bid on a lower price (derivatives), right? So, who are all those crazy people bidding only in 1 direction (higher prices)?
