There is info out there about the fallen, and you could try putting that info together, seeing what you get. I cant help but think, they may have been the last race the traveller used his light on, but during there great war they used darkness to help them selfs win, (think that's what the servitors are) hence why the traveller moved on and they became fallen. Fallen have been seen helping cabal and vex and us, but all ways fighting hive or trying to contain them, so there still loyal to the fight, but just very bitter at the other races. with us it could be because the traveller picked us? cabal because they still have a home world and darkness not got to them yet? vex maybe its one of the fallen house made them and sent them back? I think when playing alpha that the fallen worship there servitors as gods, and seeing how there shaped like the traveller but dark instead or white, and that is what makes me think it was there downfall.