The movies in the "Kelvin timeline" and the show Star Trek: Picard include references to a supernova that destroys Romulus and kills most of the Romulan population.
Why don't the Romulans see this coming and evacuate ahead of time?
Supernovas generally don't happen quickly. There's usually plenty of warning. In TNG S04E22 "Half A Life", Lwaxana Troi even comments on this to a member of a species that is much less advanced than Romulans. He answers that they were reluctant to abandon their home, but it's clear they definitely saw it coming. So you'd think the Romulans would know...
Was there some sort of unique event that caused the supernova to be a surprise?