하필 + time/thing: 하필 오늘 · emphatic: 하필이면 (왜 저예요?)
of all (times/things/people), why … of all
Example
소풍 가는 날 하필 비가 왔어요.
Of all days, it rained on the day of the picnic.
Word parts
하 what, why+필 must
Literally 'why must it be': 'what' + 'must'.
Good to know
Complains about bad luck or timing. 하필이면 is more emphatic: 하필이면 왜 저예요? 'Why me, of all people?'