Apparently, Ukrainian drones pushed through and started a chain reaction.
Explosions reportedly continued for hours, and authorities evacuated nearby settlements. Initial reports indicate that the site, previously protected by one of Russia’s densest air defense networks, suffered catastrophic damage.
Russia has a long history of open storage at these sites. They also lost a ton of bunkers a few months ago at other sites. So they likely did not have much of an option, and they chose open store it at their “best defended” base.
I personally would bet that site was overstocked as it was likely the primary ammo dump by default. All of the newly manufactured missiles and shells going there directly from the factories.