A good way to hold this word: picture a fire that is already burning steadily. Fat drips on it, and for a second or two the flame leaps up — then it drops back to where it was. Everything flare-up is used for keeps that shape. Something was quiet, something jumps, and the jump is understood as one stretch rather than a new permanent state. That is why English counts them — one or two flare-ups — and why the calm before and after is always part of the picture.
From the word’s history
Wiktionary records flare-up as deverbal from the phrasal verb flare up, and notes the word was originally used of flame and only later of any violent activity or emotion.
Wiktionary 記錄 flare-up 是從動詞片語 flare up 直接變成的名詞，而且註明它原本講的是火焰，後來才擴大到各種激烈的活動或情緒。所以把火的畫面放在最前面，不是我們編出來的比喻，而是這個字自己走過的路。
Wiktionary entry for flare-up (kaikki:3f6f69a190036ae4:1443425983:8646): "Deverbal from flare up"; sense 0 gloss "originally of flame, but now used more generally of any violent activity or emotion".
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