It was unknown what had started the fighting, but Qyburn reported that in the end, everyone with a weapon in hand were fighting each other. Somewhere along with the chaos, a building had been set on fire and it had spread. That's when the terror began, as almost all of King's Landing was quickly engulfed by wildfire. Every important square, including Rhaenys and Visenya Hill, had been engulfed in wildfire as well. Every gate was also burning in the jade-like flames, trapping everyone inside the fiery hell that was King's Landing. Cersei knew that everyone in the Red Keep would have shared that fate of the people outside, if not for the accidental discovery and removal of the caches of wildfire below the Keep some time ago.