Trata-se da inversão inglesa.but-not-only-did-that-not-happen-por-que-not-duas-vezes-t56929.html
Usually, we put the expression at the beginning of the sentence to emphasise what we're saying. It makes our sentence sound surprising or striking or unusual. It also sounds quite formal. If you don't want to give this impression, you can put the negative expression later in the sentence in the normal way:
Seldom have I seen such beautiful work.
('Seldom' is at the beginning, so we use inversion. This sentence emphasizes what beautiful work it is.)
I have seldom seen such beautiful work.
('Seldom' is in the normal place, so we don't use inversion. This is a normal sentence with no special emphasis.)
Here are some negative adverbs and adverb phrases that we often use with inversion:
Hardly Hardly had I got into bed when the telephone rang.
Never Never had she seen such a beautiful sight before.
Seldom Seldom do we see such an amazing display of dance.
Rarely Rarely will you hear such beautiful music.
Only then Only then did I understand why the tragedy had happened.
Not only ... But Not only does he love chocolate and sweets but he also smokes.
No sooner No sooner had we arrived home than the police rang the doorbell.
Scarcely Scarcely had I got off the bus when it crashed into the back of a car.
Only later Only later did she really think about the situation.
Nowhere Nowhere have I ever had such bad service.
Little Little did he know!
Only in this way Only in this way could John earn enough money to survive.
In no way In no way do I agree with what you're saying.
On no account On no account should you do anything without asking me first.