To be slightly more precise, a Tauni covert team went poking an old Harmony base for evidence to present to us about the evils of the Harmony.
They botched a roll, triggering a fail deadly torpedo stockpile. The blast propagated through the base, triggering others, resulting in a sizeable portion of the moon achieving local escape velocity - but then caught in the planet's gravity well, with predictable results.
Our ships in the area were too far away to return in time to assist before impact.
In character, we know the old moon base blew up, but not why.
The ISC (another power in the area) hates the thought of antimatter munitions, so if we knew in character why this occurred, they would use this as evidence as to why they are right.
Our own antimatter munitions (photon torpedoes) only have a small window where fail deadly is even an option, but that is not how Horizon munitions work.