Here's why:

* Plate Tectonics: The Earth's crust is made up of massive plates that constantly move.

* Transform Boundaries: At transform boundaries, plates slide horizontally past each other. This movement can create friction and build up stress, eventually leading to earthquakes.

* Haiti's Location: Haiti is located on the Caribbean Plate, which is sliding past the North American Plate along the Enriquillo-Plantain Garden fault zone.

Important Note: While the main earthquake was caused by movement on a transform boundary, the region also experiences seismic activity related to the nearby subduction zone where the Caribbean Plate is being forced beneath the North American Plate.