Historians tend to cite two major factors that led to this growth.
First, there was the factor of the climate. The average temperature for the region rose slightly after 1000. This made for a longer growing season which meant more food. This allowed for a greater surplus that could be traded. It also meant that more people could be fed, which would allow more artisans to live in towns and cities, living off the excess food produced.
Second, there were technological factors that allowed for greater food production. The heavy, wheeled iron plow was a major factor, as was the horse collar that allowed horses to pull the plow. Horseshoes were another innovation that allowed the plow to be used. Advances like this also contributed to the increase in agricultural production.