My (fanfic) money is still on someone within the Pact causing it (accidentally), then scrambling to pin the blame on the Federation. Note that the supernova (2387) happens just a few months after the Section 31 exposé (late 2386), so the Federation's reputation would be at an all-time low. The Pact could use this to prevent Federation aid from entering Romulan space (helped along by the Federation's own swing toward isolationism [echoing the Picard timeline]), further putting the Romulan successor-states at their mercy --and giving them casus belli to declare war.
(Destabilizing Romulus also removes the wealthiest power within the Pact, allowing the Tzenkethi to gain influence [the Kinshaya and Breen are hurting after their disastrous attempts to meddle in Klingon politics, and the Tholians mainly joined this thing to get everyone to leave them alone], and leaving the Gorn as the sole moderate voice, easily drowned out.)
It falls to certain heroic Starfleet crews (and two renegade Tal Shiar agents) to uncover the truth and prevent a full-scale war. (Meanwhile, Wesley, Spock, and 11-year-old Rebecca Sisko are the only ones who can prevent spacetime itself from being torn apart; sacrifices will be required.)
The Tzenkethi were shown to be experimenting with artificial wormhole tech to try and counter Starfleet's use of Quantum Slipstream, and of all the Pact members they probably have the least regard (being that even their own internal hierarchy is based on genetic "superiority" determining one's place in society) for alien species.
I can't remember the novel, but I remember specifically one instance where a character was speculating with some horror what might happen if an artificial wormhole were to be weaponized and used to destablise a star, somehow. IMO, they were hinting there that it was ultimately the Tzenkethi that brought about Hobus.