Picture something that was up coming slowly back down to a flat, quiet level: a swollen piece of ground, a wall of flood water, a hard wind, a wall of noise, a hot temper. Subside is that downward settling. Two things stay true every time. It happens by itself, with nobody pushing it down, which is why the verb never takes an object. And it happens slowly, so you are usually looking at the stretch of time just after the highest point.
From the word’s history
Wiktionary traces subside to Latin subsīdō, glossed there as to settle, subside — a word about sinking down and coming to rest. Every modern use keeps that picture, whether the thing sinking is ground, water, weather or feeling.
Wiktionary 說 subside 來自拉丁文 subsīdō，意思是「沉下去、安定下來」。今天不管沉下去的是地面、水、風還是情緒，畫面都還是同一個：往下，然後靜下來。
Wiktionary, English 'subside' (packet etymology_text; source_ref kaikki:3f6f69a190036ae4:668043596:30210)
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