Do note that i dont know this for sure, but my guess is it would go in order in which each player hits the boss. Based also on how fresh drop list is. New list all drops in play low chance good loot unless really lucky. If Left up to complete randomness and luck kill as many kills are made with few or no unique drops then chance is really high for good drop.
You times as you can kind of and you will see this in double events you get drops for 2 mobs in each kill. Iv noticed when you get good drops often its the exact same drops in corpse as previously.
Other bosses totally random and some are more drops to most dmg healing or dmg received. Percht Queen however is ment to be easy and hp is really low as it is. Again though this is all my assumption and in no way fact.the drops you want :D