Picture the moment after a big explosion. The blast is over and the noise has stopped, but dust was thrown high into the sky, and it is still up there. Minutes or days later it begins to come down — on roofs, on fields, on people who thought the trouble had finished. Fallout can be thought of as that late-arriving dust: not the event, but the part of the event that lands on you afterwards.
From the word’s history
The supplied Wiktionary entry records fallout as a deverbal noun formed from the phrasal verb fall out (fall + out) — literally the stuff that falls out of the sky.
手上的 Wiktionary 資料寫著 fallout 來自片語動詞 fall out（fall + out），是直接由動詞變成的名詞。字面畫面就是「從天上掉出來的東西」，這條線索跟今天兩個意思都對得上。
Wiktionary entry for fallout, kaikki extract source_ref kaikki:3f6f69a190036ae4:1116534998:32348 — etymology_text: “Deverbal from fall out; fall + out.”