This is so old haha, they made put that in the cave, because that was the old loot cave, and when they killed it (removed it) they added that in as like a reference. The Part where the enemies were moving together was a coincidence, that is like a public event, except it's a huge war of fallen and hive. The Husk of The Pit dropping was also a coincidence, you should keep that gun, it eventually upgrades to an exotic.