extreme monster spawning in T16 full mirage

have done about 200 T16 full empowered mirage while doing ambush box strat. Just did one where thousands of mobs spawned outside of the boxes inside the mirage, to the point it covered the whole map, talking thousands. thought it might be the boxes without being opened spawned their mobs both when box gets opened and when entering the mirage. Not sure tho. Their drops seamed to be regular and the map only had 50% pack size with all extra mods, picked reflected mist as mirage reward if that matters