La Nina is basically the reverse of El Nino. During a La Nina episode, the waters cool off the Peruvian coast. Hurricane activity generally increases during this time. La Nina episodes can also last for a year or more. El Nino conditions cause tropical wind fields to increase in the upper levels of the atmosphere.
Strong winds are detrimental to the development hurricanes from thunderstorm complexes.
The strong winds literally shear off the tops of potential hurricanes. This shearing cuts off much of the convective power in a thunderstorm complex and it subsequently weakens. This may occur for the entire hurricane season. During La Nina, tropical wind fields revert back to rather low velocities. Therefore, developing hurricanes have much more favorable conditions for development.