。
She quit her job for some reason.
Literal
She [topic-は] some reason [for-で] job [object-を] quit.
何かの + noun is a slightly different shape from 何か + noun on its own — the inserted の treats the noun as a category, giving 'a [N] of some kind.' Here 何かの理由 lands as 'some reason or other,' the speaker explicitly disclaiming knowledge of which one. The で marks the reason; やめる is the everyday verb for quitting a job, school, club, or habit.