Over production led to an increase in reserves and a decrease in price. Hop prices actually were so low, it was no longer profitable to plant fields in N.America, so everyone stopped. A few years later, the reserves dried up, but noone had been planting hops for a few years, and as supply fell, prices rose.
Combine this will an absolutely terrible growing season in Europe where Hallertaus came in at less than half the IBU they usually come in at, and they got I believe about 1/3 of their harvest due to late season storms.
England has been cutting back on growing for a while now. The very popular English varieties like Kent Goldings and Fuggles are in high demand, and they're just isn't a lot to go around.
So these combined into the perfect storm last winter, the harvests were aweful, the ones harvested didn't stack up, the harvests were smaller than usuall, growers had switched to more profitable crops, and reserves ran out.
This years harvest was better, but it takes a couple years for new fields to produce, so all that were planted this year to combat the shortage, won't be ready till next. The hops from Europe came in better this year too with a better yield. The shortage has gotten much better than it used to be, and by next year when the new fields start to really produce, it will all be better.
EDIT: Revvy, that let me google it for you thing is cool.