Something was up on top of something else and held there — a rider on a horse, a camera on a tripod, a disk inside a system. Dismount is the undoing of that: it comes back down, and the support lets go. The picture always has two halves, the coming down and the release, and different uses of the word lean on different halves.
From the word’s history
The word is dis- (undo, reverse) plus mount (to climb up onto something). Probably taken straight over from Old French desmonter. So the reversal is built into the word itself: whatever was mounted comes back down.
這個字是 dis-（解除、反過來）加上 mount（爬上去、架上去）。可能是照著古法文 desmonter 直接搬過來的。所以「倒回來」這件事就寫在字裡面：原本被 mount 上去的，現在下來了。
Wiktionary entry for dismount (kaikki:3f6f69a190036ae4:510038219): From dis- + mount, probably a calque of Old French desmonter.
登入就可以看每一支怎麼連回核心畫面。