The Traveler was attacked by Rasputin because he sensed it was getting ready to abandon us like it did the Eliksni(aka the Fallen). That's why its bottom side is all damaged. Knowing it could not outrun the darkness with the damage it sustained it Released that wave of light and created the ghosts to help protect itself and fight the darkness.

I think the Traveler is obliged to stay with humanity since his sphere was kinda destroy in the previous wars with the fallen( twilight gap war) and so he cannot travel anymore. Also that's the reason the Traveler is dormant right now. Check what the Speaker has to say.

I wouldn't be so sure... Rasputin shot the traveler, and screwed over Charlemagne, all in the name of saving his own skin... the Traveler figured out that because it couldn't run, it needed to do something to try and ensure the ideals of the Light, the Sky lived on. ... which became the Ghosts and their Guardians. ... The Traveler was GOING to leave and Rasputin basically said "oh [i]hell no[/i] you don't." The traveler and whatever live(s?)(d?) inside it never wanted to play by the hostile rules of the Dark, which the easiest description is "do [i]anything[/i] to survive an be on top." The Traveler was forced to do something differently this time, or it would have left us high and dry too... and then we'd be like the Taishibethi bird people...