In the Skywatch, near where the two helicopters reside there is a cave that is a spawn point for the Fallen. If you are able to kill the initial spawn of enemies before you can come out the cave then the game will keep sending the enemies out. Me and a group of 4 other Guardians went chest hunting and caught a spawn of Fallen before they left the cave. We killed them, and turned our back to the loot chest inside. We were promptly stab by the melee of the fallen dregs and a few captains. We turned around and destroyed them and the wave after wave of fallen that kept coming effectively setting up a chain of light orbs that let us keep our supers over and over again. In a period of 30 minutes we had killed enough fallen to get me 2000 glimmer and about 13 encrypted engrams. Ill add a picture of the aftermath later.
This wasn't just part of the big battle that the Hive and the Fallen occasionally have at Skywatch, was it? I don't believe it happens very often, but when it does, you get a load of spawns from both sides.