The Fallen are named Fallen as there was a great collapse in this once great species. Similar to us really. The Fallen were the original "owners" of the Traveler, they worshipped it, but then the Traveler saw they weren't strong enough to keep the Darkness away and abandoned them. Their golden age was destroyed and the once great species suffered a huge collapse. They are now space pirates looting to survive, therefore, their name probably came from there.