Full Idea
Necessity is supposed to be the cause of something coming to be: in truth it is often only an effect of what has come to be.
Gist of Idea
Necessity is thought to require an event, but is only an after-effect of the event
Source
Friedrich Nietzsche (The Gay (Joyful) Science [1882], §205)
Book Reference
Nietzsche,Friedrich: 'The Joyful Science, and 1881-82 fragments (v 6)', ed/tr. Del Caro,Adrian [Stanford 2023], p.151
A Reaction
This sounds like an account of the traditional idea of destiny - which sees inevitability in some major event, which was previously unpredictable.