。
She kept losing her temper.
Literal
She [topic-は] constantly snap-became.
カッとなる is the standard expression for snapping in anger — flying off the handle. The mimetic カッ describes a sudden flare-up, a hot flash of temper; combined with なる ('become'), it captures the moment of losing control. The past tense しょっちゅうカッとなった describes a habitual pattern in the past — she repeatedly hit this state, suggesting chronically poor temper rather than a one-off incident. カッ is often written in katakana, as here, for visual emphasis on the suddenness; the more conventional spelling is the hiragana かっと.