This is called a "Shower" (Bungies name for it anyway) I ran into one of these the night the famous loot cave was patched, and I got like 4 or 5 Blue Engrams and one Purple, whereas a friend got 2 Blue and one Exotic! In the same battle. They are very very very rare! I play for about 3-5 hrs a day and a good portion of that time is spent farming. I found this in the Rocketyard on Earth.