。
She has a tendency to throw tantrums.
Literal
She [topic-は] tantrum [object-を] tend-to-cause [is-である].
~がち ('tend to, be prone to') is a productive suffix for habitual or characteristic tendencies, usually with a slightly negative or unflattering nuance — 忘れがち ('prone to forgetting'), 病気がち ('sickly'), 遅れがち ('tends to be late'). It attaches to the masu-stem of verbs or directly to nouns. The clause-closing ~である is the literary copula, giving the sentence a slightly elevated, written-leaning tone — appropriate for a personality assessment in a measured register. かんしゃく (癇癪) is a 'fit of temper, tantrum,' often used of children but applicable to adults who lose control. かんしゃくを起こす is the standard collocation for 'throw a tantrum.'