go off the rails
US /ɡoʊ ɔf ðə reɪlz/
UK /ɡoʊ ɔf ðə reɪlz/

1.
誤入歧途, 脫離正軌
to start behaving in a way that is not acceptable or expected, especially in a wild or uncontrolled way
:
•
After his parents' divorce, he really started to go off the rails.
父母離婚後,他真的開始誤入歧途。
•
The project seemed to go off the rails when the main manager left.
當主要經理離開時,項目似乎脫離了正軌。