Minerva slaps Trujillo primarily because he gropes her while they are dancing. She has plenty of other reasons to do so as well, however.
Minerva and her family have been invited to a Discovery Day party at one of Trujillo's "party mansions," supposedly the place where Trujillo keeps his favorite mistress, but the house is unoccupied at the moment. Minerva has been invited as a pretty girl Trujillo would like to "try out." When they dance together, Trujillo is openly sizing Minerva up; he handles her roughly, steps on her feet, calls her a "national treasure," and allows his eyes to rove all over her body. When Minerva brings up the idea of studying law in the capital, Trujillo is dismissive, then suggests that if she were to come to the capital he would be able to see her more often.
Things take a turn for the worst when Minerva insists that she cannot be with any man until she finishes her studies. She mentions her friend Lío, a revolutionary who has been arrested, and Trujillo becomes angry. He knows the name and begins to recognize that Minerva is not just another woman for him to "conquer," but a political opponent. When he aggressively grabs her and grinds his groin into her, she reflexively slaps him.