Usually due to supply and demand.
A couple of years back SD Ram was dirt cheap and one factory in the north east closed as it was losing money making the chips for the manufacturer that they supplied.
Shortly afterwards a factory in the far east was gutted by a fire, leaving a shortage of chips worldwide
The price of memory went through the roof for a few months before settling back to its normal price again.
I dont know what caused the increase in DDR last year but its time it dropped back as its dearer now than it was some 12 months ago