As soon as I opened the soda can, the liquid inside sprayed all over the floor. 請問此句為何不是用被動語態was sprayed .,而用sprayed,可否幫忙說明一下?

spread屬action verb（動作動詞），可以當「及物動詞」或「不及物動詞」。主詞是「物」，在做擬人化描述時，可以當動作動詞的主詞，如：
The fields spread out before us.=田野在我們面前展開。
The ink spread over the desk.=墨水在桌面上滲開。

As soon as I opened the soda can, the liquid inside sprayed all over the floor.
＝我一打開汽水罐頭，裡面的液體立刻噴得滿地。
因為是汽水裡的氣體讓液體自己噴出的，不是我灑的，所以用spread。在這句裡，液體被擬人化了，自己可以做出行動來。

Some diseases were spread by flies.=有些疾病是蒼蠅所傳播的。（這裡的was spread是及物動詞被動態，句子的主詞是原來的受詞，by後面是邏輯上的主詞。）

以下的資料是2017/2/25在字典裡找到的，請參考：
If something "spreads" or "is spread" by people, it reaches or affects a larger and larger area or more and more people.
在這種句子裡的動詞spread是被字典定義為「作格動詞」（ergative verb）。
see p.1507 "spread" Collins Cobuild English Dictionary for Advanced Learners

An ergative verb is a verb that can be both transitive and intransitive, where the subject of the intransitive verb is the same as the object of the transitive verb. For example, 'open' is an ergative verb because you can say 'The door opened' or She opened the door'.
see p.519 "ergative" Collins Cobuild English Dictionary for Advanced Learners

Ergative verbs allow you to describe an action from the point of view of the performer of the action or from the point of view of something which is affected by the action.
see p.xxxi V-ERG, Collins Cobuild English Dictionary for Advanced Learners

所以... the liquid inside "spread/was spread" 都是正確的（在邏輯上，主詞都是動詞的受詞）。
請「忽視」我先前以主動／被動來做的錯誤解釋，不要被我誤導了。抱歉。

前面名詞有動詞銜接了 就不需要助動詞