There would have been a line of text appearing saying something like "The enemies are moving against each other..."
Usually a great time to pop the consumables and farm glimmer (and engrams...) like there is no tomorrow :D
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It's only happened to me once & I've had this game since Sept 9, at midnight. I thought it was something they added last patch/update.
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I've run into literally dozens of these; they are great fun and a nice break to the grind when farming mats (especially Spinmetal around the entrance to Devils Lair) There is another 'instance' for this to happen by the old loot cave; stack loads of Hive and Fallen flooding out and murdering each other and/or anything caught in between :)
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It happens to me all the time
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Must not have noticed the text.