Suicide Squad: 15 Major Reveals From The New EW Details

2. There's A Trigger Event

Though Waller suggests the idea of Task Force X to the government to ease their fears over alien invasion, she is rebuffed initially, and as EW say "until Midway City is threatened by a powerful mystical enemy, and Waller needs to activate the whole squad of prisoners."

So, we're either looking at the suggestion that Enchantress kicks off on the city and the Squad are forced to intervene, or something else does.

Personally, I think Waller engineered the situation to her benefit, unleashing Enchantress in order to get her own way. And the other big bad - presumed to be Tattoo Man - comes later, perhaps as part of Joker's attempts to mess with the Squad's mission?