One picture can hold the whole word. A feeling arrives like a wave, the body cannot stand up under it, and the person goes down. In the older use the person really falls to the ground. In today's use nobody falls, but the same , helpless feeling is what swoon points at — so a fan screaming at a concert and a queen dropping at bad news are the same picture, one literal and one borrowed.
From the word’s history
Old English swōgan meant to make a sound, to overrun or to suffocate; its past participle ġeswōgen meant insensible or senseless. Middle English swounen came from that and meant to faint.
古英語的 swōgan 有「發出聲響、覆蓋過去、使人喘不過氣」的意思，它的分詞 ġeswōgen 就是「失去知覺」；中古英語的 swounen 從這裡來，意思是昏倒。被什麼東西蓋過去而失去知覺——這條線索正好接上今天的兩種用法。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1142492940)
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