I had that today.
Completely normal patrol on Earth when I decided to enter the crashed fallen ship. Once inside I managed to wander my way into the lockdown zone in Devils Layer. Thought it was cool that I could find a back door into that location.
After dispatching everyone I carried onto the walker and finished him. Now for the interesting part I was wander back through the industrial building complex and stumbled onto a one man (possibly woman) war against the fallen. Joining the impromptu firefight we spent 10+ minutes fight every kind of fallen had no idea a non scripted encounter could be so big. We must have taken out 50 fallen between us.
English
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That gets promted instead of a Public event. You'll see a message above your super meter saying "The enemies move against each other." That means get ready for war.