If you're wondering, this is towards the end of the level when you're heading for the control room.
I'm not sure, but I think this will only happen if you aren't able to get either of the banshees secured by the elites. Once you're at the top of the pyramid, after alot of walking and fighting, check the tower to see if there's any enemies. If there isn't any, then go open the door, and check again. It can trigger a whole bunch of Covenant up there.
If they do spawn in the tower, there will be:
A bunch of grunts.
Maybe an elite, or two. Not sure.
I had to find out the hard way. I had opened the door and backed up the tower ramp, and got smashed into the wall by a large shield.
Not getting a banshee could trigger a lot of enemies, so keep an eye out.