The Fallen speculation on Guardian Radio (Destiny)
Byf had some interesting thoughts regarding the Fallen in the latest episode of Guardian Radio. They're worth listening to (as is every episode), but the short version (ignoring the Myth connections) is the speculation that the Fallen were once the beneficiaries of the Traveler, as humanity was during its Golden Age.
The speculation is then that the Fallen had their own Golden Age, but the Traveler left them behind, leading to their fall. What was left of the Fallen have followed the Traveler to Earth, presumably to revive their race via regaining access the Traveler.
It's an interesting theory, especially how it parallels what humanity has gone through in Destiny. It fits what we know of the Fallen and explains their motivations.