Optimus was the only person holding the Titans together, so when he blasted off into space, everything fell apart. For a while the remaining Titans stayed together for Optimus's sake, hoping he would come back, but when he didn't, they went their separate ways. Nebula went to learn magic from an old friend, while Chaos went looking for Optimus. only Brahma continued his duties, with a minor change, he did it from the throne of a newly created planet. Reaper watched all this unfold and felt that the Titans needed intervention. Intervention the likes of which the Titans couldn't provide. Intervention only providable by one person. Prime.