필유곡절이 · 필유곡절을
there must be a reason behind it
Example
그가 갑자기 그만둔 데에는 필유곡절이 있을 거예요.
There must be some reason why he suddenly quit.
Word parts
필 surely+유 have+곡 bend+절 break
Literally 'there must be twists and turns' behind it.
Good to know
곡절 is common on its own too, as in 우여곡절 'ups and downs'.