Pretty well sums it up...why does the price of oil go up? Because they can...same reason a dog licks himself.
Price flucuation in all commodites is controlled by those that manufacture/own the commodity.
I purchase a very large amount of forest products for my business...when hurricane season is imminent, plywood, osb prices spike months before it hits. Why because they can.
When there is a surplus on the market they "shut" down production to get the price they want. We see upwards of 30% spike in prices thru the year...makes it kinda tough to quote a year out.