The "historical reasons" Dominicans give for hating Haitians is bullshyt. As far as I can tell the Haitians did nothing wrong except go to war against cacs and their like. The Dominicans used anti-black libel to justify massacring the Haitians:
The Haitian occupation of Santo Domingo (1822-1844), although passively accepted by most of the population (and even celebrated by lower-class groups), was strongly rejected by the elites, who lost their privileges and administrative jobs to the occupation armies. Dominican elites further resented being at the mercy of individuals whom they considered inferior because of their skin color and social status. The great majority of Haitian army officers were ex-slaves themselves, with little or no education, and lacked the finesse and manners that elites regarded so highly. During the period of the Haitian occupation, many of these elite families left the country, a fact deplored by Joaquín Balaguer, who commented that Santo Domingo lost most of its "best" families at that time (Balaguer 1984, 59-60).
When the Dominican Republic became independent in 1844, elites portrayed this event as the realization of their efforts to maintain Hispanic-Catholic culture intact in the face of the Haitian occupation. As they stated in the independence manifesto of the Dominican Republic, "due to the difference of customs and the rivalry that exists between ones and the others [referring to Haiti and the Dominican Republic], there will never be a perfect union nor harmony" (Despradel 1974, 86).
With the Haitians out of the scene, Dominican elites regained their privileged social position and their high-level administrative posts.
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In other words, the Dominicacs are still mad the "inferior" black Haitians ruled over them.