To see this glitch, start the level The Silent Cartographer on any difficulty, preferably Easy. Play through the level as normal, except go straight to the security system instead of the Silent Cartographer. Once you are at the Silent Cartographer and the door is already open, go straight through and spare at least 2 or 3 Covenant on the top level.. Once you have activated the Cartographer, go back up until you reach the final ramp to the surface.(This is easier in a Warthog.) Once there, draw some Covenant over at the top of the ramp by firing your weapon. Then quickly run to the top of the ramp. As the Stealth Elites spawn, all previous Covenant there will suddenly die.
It isn't known why this happens or why it needs to happen.
It has no effect to the Covenant still in the facility.
It could be the same effect that kills the marines.