。
She has a tendency to suddenly fall into a slump.
Literal
She [topic-は] suddenly become-depressed tendency [subject-が] exists.
ふさぎ込む is a compound: 塞ぐ ('to block, shut up') + 込む ('go deeply into'), giving 'shut oneself in (mentally), withdraw into a depressed state.' It's the verb of choice for dipping into a heavy mood — heavier than 落ち込む ('feel down') but less clinical than 鬱になる ('become depressed' in the medical sense). 性癖 means 'inclination, propensity,' with a slightly old-fashioned, even cautionary tone — modern speakers often use 癖 ('habit, quirk') or 傾向 ('tendency') in everyday speech. Note the layering of suddenness: 急に modifies ふさぎ込む, characterizing the *manner* of these mood-dips even as the predicate ある describes the *general tendency*.