One way to hold this word: a big open wagon at the front of a parade, with a band playing on top of it. The music pulls people out of their houses, the wagon keeps moving, and a crowd forms behind it. Two things then become sayable at once — the moving crowd itself, and the act of running after it and climbing aboard. Every use of the word is one of those two, and the wagon is always still rolling.
可以想成遊行最前面那輛開頂的大車，上面坐著一隊樂手在演奏。音樂一路傳出去，把人從屋子裡拉到街上。車不停地往前走，後面就聚出一群人。從這個畫面可以講兩件事：一件是那群跟著走的人本身，也就是「風潮」；另一件是追上去、爬上車的那個動作，也就是「跟風」。這個字的每個用法都是這兩件事之一，而且車永遠是正在動的。
From the word’s history
Wiktionary gives simply 'From band + wagon' — a wagon carrying a band — and records the literal parade wagon as the word's first noun sense, with the movement sense marked figurative.
來源證據只寫了一句：來自 band 加 wagon，也就是「載樂隊的車」。它把字面的遊行樂隊車列為名詞的第一個意思，而把「風潮」標成比喻用法。所以從字面到比喻的這條線，是來源本身就排好的。
Wiktionary, "bandwagon"
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