If there is one theory I do like as to why the Romulan sun would go Supernova, it's because I think a remnant Jem'Hadar / Dominion faction decided to detonate it as payback for the Romulans going onto the Federation side during the Dominion war. The Dominion were doing fine until the Romulans tipped the scales. As payback for losing the war, they cause genocide on Romulans home planet.
Plus the founder impersonating Bashir had similar tech that can artificially cause a Super Nova. It makes sense that a remnant Jem'Hadar / Dominion faction can somehow get access to such a device.