Star Trek: Picard offered Star Trek fans their first glimpse into the Prime Timeline after the Romulan supernova (that led to the Kelvin Timeline's creation). While it addressed issues like the Romulan diaspora that occurred after the destruction of Romulus, it didn't get into the matter of what caused the supernova to happen in the first place. While unexpected, most assumed that the natural phenomena caused the supernova. The Star Trek: Picard tie-in novel The Dark Veil suggests that may not be the truth. SPOILERS follow for Star Trek: Picard: The Dark Veil by James Swallow.
The Dark Veil focuses on the USS Titan under Captain Riker's command during the years after the attack on the Utopia Planitia Shipyards and Jean-Luc Picard's resignation from Starfleet but before the Romulan supernova. While official cooperation between Starfleet and the Romulan Star Empire is over by the time the story takes place, Riker and the Titan crew find themselves working together with a Romulan ship to avert a catastrophe near the neutral zone between Federation and Romulan borders.
At least most of them are working together. A Tal Shiar agent named Helek aboard the Romulan ship is secretly a member of the Zhat Vash. She's working towards her secretive purposes, and she has an agent loyal specifically to her aboard the vessel. That agent is a scientist named Vadrel, or at least that's the name he uses now. The novel implies that Vadrel was once someone else, but that he got involved in a scientific project with such dire repercussions that he had to abandon his old life and take on a new identity. The Tal Shiar helped him do that, and because Helek knows Vadrel's secret, he has to do her bidding.
The novel doesn't give specific details about Vadrel's project, but it sure sounds like it could have led to the Romulan supernova. Here's some of the text:
"In his life before this one, the man who was now Vadrel had been a part of a project so secret it defied even the most byzantine of Romulan schemes. But the fires of its failure had condemned billions to certain death, and he shared a great deal of the responsibility."
It is hard to imagine anything but the supernova leading condemning billions to certain death. The book later offers some more vague insights into Vadrel's past, implying that whatever he was involved in had direct approval from the Romulan Praetor:
"He had once been part of an elite cadre of thinkers who crossed the limits of what was possible, while other scientists had wrung their hands and bleated about ethics and accountability. Unshackled from all restrictions by the personal command of the praetor himself, for the advancement of the Empire, they had been at the cutting edge of Romulan sciences.
"Forbidden, volatile technologies like omega particles, protomaterials, time-active substrates, even red matter, all of it had been at his fingertips… But then there had been the error, and everything else that followed. "0comments
It isn't confirmation, but this certainly sounds like the Romulan government was responsible for a scientific project that caused the system's sun to go supernova. Whether or not this is ever explored further in Star Trek: Picard or another Star Trek series remains to be seen.