It's one of the big not-explicitly-stated things of Starsector, and it's seen uncountable discussion threads. Looking at the mission texts (among other sources), though, it seems to very strongly hint at Tri-Tachyon being behind it.
- Sinking the Bismar: Tri-Tachyon is explicitly stated to have psychological profiles on Hegemony officers, and to plan around their expected behavior. This seems like a very conspicuous inclusion, and I think the game's writers are fond of using small events to foreshadow larger ones that follow from the same causes.
- The Diktat description states that "Andrada's Hegemony psych profile describes him as "intelligent, charismatic, yet prone to narcissistic excess". Andrada is the only other character to have a psychological profile mentioned, after the possibility of Tri-Tachyon psychoanalyzing Hegemony command staff was set up in the previous item. Creating a crisis on his watch to provoke an overreaction seems like a natural thing to do.
- Forlorn Hope: The description conspicuously mentions a phase cruiser carrying a planet-killer device. While it could, in isolation, just serve to characterize Tri-Tachyon as amoral people who are willing to kill hundreds of millions from orbit to win a war, the explicit inclusion of the information that the PK-carrier was a phase ship, combined with the focus on the mysterious and unattributed destruction of a planet later on, feels calculated.
- The Historian covers pretty thoroughly why Opis's destruction was not what Andrada would have chosen as the catalyst for his own mutiny. "Andrada took a portion - but only a portion - of the Hegemony navy in the course of his mutiny" and "immediately faced with the catastrophic consequences of the Opis incident, ongoing rebellion, complete lack of logistics, and the pressing need to maintain a credible military force against a vengeful Hegemony". It's stated elsewhere that a majority of the Hegemony fleet on hand joined with Andrada over the Hegemony, and reasonable to expect that he could have taken more supporters with him if he had planned things out, or kept the planet-killer as insurance, or chosen to catalyze a mutiny without preemptively destroying what was about to be his.
- Predator or Prey?: Daud's career path to Hegemon involving the command of a cruiser and its escort tells the audience that a successful military career is at least one route (though it is implied to be the predominant one) to positions of extreme authority. Casting Andrada as the perpetrator because of a lust for power raises the obvious question of why he didn't remain in place and seek authority over a much larger military power, especially since his career path at the time appeared to be pointing quite clearly in that direction.
- The Kinetic Blaster description mentions that there has been, "with near certainty, TT technical assistance to Sindrian Mutiny.". This felt a bit strange at first sight - the Diktat's main supplier had been the League, and their special weapons program was not militarily significant enough to take on as an act of charity to an enemy-of-my-enemy. On the other hand, promising potentially world-changing weapons to Andrada fits well with the sense of grandiosity established by his psychological profile, giving Tri-Tachyon an "in" with the leader they created, in addition to supplying them with a politically convenient deniable beta-tester for their reverse-engineered Omega weapons and, doubtlessly, additional income.
- The Timeline places the Askonia crisis right before the Second AI War. Removing Andrada from the equation seems like a vital prerequisite for starting a new war with the Hegemony, given that Tri-Tachyon's military power was already crippled by the first one, and the Hegemony had a legendary, highly-competent hero-admiral to both rally around and use as a magnet for allies.
Aside from the in-game hints, looking at the relevant parties leads us to the same conclusion through process of elimination. Tri-Tachyon is the only actor could possibly have considered blowing Opis up to be beneficial to their interests.
- The in-system parties (government and rebels) are far worse off - Opis was the system's main population center, after all, so most of their loved ones are dead, and even if Andrada hadn't taken power over them, they're now at the center of a major interfactional conflict over WMDs, meaning they've lost their power center and are surrounded by better-armed enforcers who intend to stick around until the bomb situation has been sorted out. Sovereignty and/or independence are out the window no matter how things shake out.
- The Hegemony, even if things went better for them, would have stained the reputation of their legendary admiral by associating his name with a catastrophic megadeath event and damaged their legitimacy as Domain-successors by allowing it to occur on their watch.
- The League, even if blowing up the planet had somehow brought the system into their fold, would've discarded the jewel of the system that they were fighting over, which was already considering membership to begin with.
- Andrada was the lead admiral of a major faction, and would've had a legitimate shot at becoming High Hegemon. The conflict occurred at the worst possible time for him, leaving the legendary hero with only a small fraction of the Hegemony's combined fleets (who, at the time, had idolized him) and in control of a system that had just lost its major planet.
Tri-Tachyon, on the other hand, is now the Diktat's key arms supplier, can use them to test their new weapons prototypes with plausible deniability, has supplied the Hegemony (which had been consistently foiling their efforts) with a painful distraction, and taken a significant chunk out of the Hegemony navy.