Haiti's Position on the Carrribean Plate

Haiti sits on a border of the North American plate and Caribbean Plate. It nearest a transform plate boundary, so as the North American plate moves west, the Caribbean plate moves east which causes a lot of friction that is unleashed as high magnitude earthquakes. It is also due to the strike-slip fault systems that run through northern and southern Haiti. A recent example is of the 2010 Magnitude 7 earthquake that tore apart its nation's capital and killed over 90,000 people. 

Sources:

Haiti - Tectonic Plate Boundaries

Why is Haiti Vulnerable - The Guardian