The situation with the shield and Chituari is from Secret Empire and only appears in the Ultimates via a Secret Empire tie in issue. The Chituari are attacking everything their path which forces the heroes to fight. Plus, if the Chituari are able to breach the shield, they will unleash death and destruction on humanity.
Ewing features Maker in his New Avengers run, so those who read that book would not perceive Maker as popping out of nowhere.
But the heroes themselves can't break the dome, right? It's effectively unbreakable? So why worry about Chituari so much? If these powerful heroes can't sent it, the less powerful aliens that the heroes can defeat soundly may be able to?
It doesn't work for me...