why did Superman attack them?
After dealing with a Parademon (they didn't know that's what it was) Hal suggests the talk to the guy in Metropolis that is supposed to be an alien, thinking he might have info or even be connected to the other alien. They show up, Hal tells Batman to stand back and that he'd go in there, lock Superman in a cage, and then get some answers. Then Superman knocks Hal on his ass and the issue ends with him looking kind of pissed and curious about these two new heroes.
It's a world where all the heroes are new, so none of them know each other, and this new Superman is more brash than the typical Superman we know.