。
She gets angry over trivial things.
Literal
She [topic-は] trivial things [at-に] gets-angry.
腹を立てる ('to get angry,' literally 'to stand up the belly') is one of the iconic body-part personality idioms — Japanese frames anger as something happening in the belly (腹), not the heart or head. The 立てる ('cause to stand') is transitive, contrasted with the more common 腹が立つ ('belly stands up' = 'to be angry,' intransitive). ささいな ('trivial, slight, insignificant') paints the triggers as small things — turning the sentence into a comment on temperament. The に on ささいな事 marks the trigger of the emotion. A standard observation about a short-tempered person.