It's because, in the aformentioned sentence, "hubiera" should be "habría". "Habría" is getting out of style in contemporaneous Spanish and it's being replaced little by little by the form "hubiera" which is frowned upon by grammarians.
Let's take a look at some sentences:
Si hubieras comido, no hubieras tenido hambre.
If you had eaten, you wouldn't have gone hungry.
No hubiera aprobado el examen aunque hubiera estudiado mucho.
I wouldn''t have passed the exam even if I had studied.
Some speakers may use "hubiese".
No te hubiese reconocido.
I wouldn't have recognized you.
In some Latin American countries, "hubiera" can mean "should have" or "could have", such as in:
Me hubieras llamado por teléfono.
You should have called me by phone.
Te hubieras matado. La próxima vez, ten más cuidado.
You could have killed yourself. Be more careful next time.
Me hubieras avisado donde andabas.
You should have told me where you were.