In the comics, the aliens attacked Earth because they were concerned about the growing population of metahumans.
The new trailer gives us our first idea of what drove the Dominators to drop by Earth and spread some mayhem.
Given the explosion of metahumans and superpowered individuals in the Arrow-verse over the past couple of years, it actually makes sense that a race of meta-hating aliens would attack Earth.
Their argument that they need to exterminate “the scourge” of our planet before it spreads to theirs is as terrifying as it is ambiguous, but it fits with what we know of the Dominators of DC Comic lore.