Its "just" a glitch that apears when you kill the boss, wipe asap, go back and repeat the process. Each time you stack this, the loot will glitch out and spawn. They must have killed the boss like 20 times to achieve this.
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http://destinytracker.com/dg/3909803772 He wiped 17 times over the course of 2 hours.
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All that effort for a few views right lol