You can think of trailer as whatever comes along behind, pulled by something else. It has no engine of its own: the car goes, and it follows. Hold that picture and the word opens up — the box of tools behind the truck, the little room you sleep in on the road, the home that was towed in once and never left, and the short reel of film that used to run along behind the main feature.
From the word’s history
The source records trailer as trail + -er, the thing that trails. It also explains the film sense directly: previews were once shown after the main feature rather than before it, so they trailed along behind it, and the name stayed when cinemas moved them to the front.
資料把 trailer 記成 trail 加 -er，也就是「跟在後面的那個」。它還直接說明了預告片為什麼叫這個名字：以前預告是放在正片後面播的，等於跟在正片後面跑。後來戲院把它移到片頭，名字卻沒改。
Wiktionary entry for trailer (kaikki:3f6f69a190036ae4:404947959): 'From trail + -er. The film sense derives from the fact that previews were formerly shown after the main feature, rather than before as is usual today, that is trailing after the main feature.'
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