one thing leads to another
US /wʌn θɪŋ liːdz tu əˈnʌðər/
UK /wʌn θɪŋ liːdz tu əˈnʌðər/

1.
一环扣一环, 一来二去
a series of events, each caused by the previous one, often leading to an unexpected or unintended outcome
:
•
We met for coffee, and then one thing led to another, and we ended up getting married.
我们喝咖啡时认识的,然后一来二去,我们就结婚了。
•
He started with a small investment, and then one thing led to another, and now he owns a huge company.
他从一笔小投资开始,然后一环扣一环,现在他拥有一家大公司。