You can picture a floodgate as one heavy gate in a dam wall, with a whole lake pressing against the other side of it. While the gate is down, nothing moves and the lake stays where it is. Lift it, and everything that was waiting arrives at once, fast, and not in any order you chose. Every other use of this word is that same gate, with something other than water behind it.
From the word’s history
Wiktionary records floodgate as Middle English flodegate, that is flood plus gate. The compound is transparent: it is literally the gate for a flood, which is exactly the picture the figurative uses borrow.
Wiktionary 記的是中古英文 flodegate，也就是 flood 加 gate。這個合成詞字面很直接——擋洪水的那道門，而後來所有的比喻用法借的就是這個畫面。
Wiktionary entry for 'floodgate' (kaikki:3f6f69a190036ae4:1386668231:10724): 'From Middle English flodegate, flodgate, flodeyate, floodȝate, flodȝete, equivalent to flood + gate.'
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