I was playing with two friends trying to get some nice loot in the Skywatch, and we headed over to this cave after doing the same thing over and over for a bit, and the Fallen kept spawning over and over again. We've only had this happen once, but this should be looked into, as it gives SUCH easy experience and could be exploited.
It's not a glitch. You happened to be there when an all out war started. Usually last 10-15min and they meet in the middle where there are tons of high level officers and a lot of minions running around. I've seen this a couple times and it's a blast to battle in.