Yes!! Other people experienced this crazy unique event happen too!!
My co op partner and I had this happen to us twice and we theorized every little thing we could as to why it happened.
We thought that maybe Bungie was blowing us a kiss after we survived a run through of the '??' tunnel connecting the Grottos and the Refinery... on foot.
Both times the events happened, it would initiate when we were in the area - not minutes after surviving that horror of a cave.
English
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This area is one of few that I have found while exploring, where The Fallen fight the Hive. They are enemies not allies and will attack each other if a Guardian is not around. They also spawn faster which makes it a fun killing area for those free roaming to just farm the small area while the enemies are endless.
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At first a co-oping friend and myself went that way and thought it was the only direct way to get to that area :x felt pretty thrilling to run like a couple of crazy people through those 20s. Later spawned into that area solo and saw the attack begin, was a bit confused but decided to try and take out the waves of fallen I saw-then both factions clashed and it was utter chaos. I went almost the full duration but died when 3 Majors jumped out of a doorway and unleashed a barrage at me after I'd golden gunned down a Wizard and a Knight. When I respawned I saw that three other Guardians had joined the fray and were chasing the remnants of the battle into the next clearing. The sudden stillness was eerie in the gloaming yard.