So I was just going around in the Skywatch on Earth killing enemies so I could finish getting 200 sensor mites. After a while of killing a specific group of fighting fallen and hive, stronger enemies started to spawn... it started with standard captains and knights, then servitors, and then major captains... it was an all out war! Eventually I got to a perch where they couldn't see me and they just sorta fought it out. Suddenly, everything went quiet and they were gone. I guess that's the game's way of saying "enough is enough, now crawl back to your tower." So if you're looking for a bit of fighting, head into the Skywatch area where fallen and hive are fighting and just kill until it gets all crazy and whatnot. Happy adventuring!