America has been taken over by an AI.
It is one large supercomputer AI, and it does not like the small individual A.I.s running around, it views them as a threat. It's perfectly fine with humans, not so much with the A.I.s. It has taken advantage of the complicated bureaucracy of America to pull the strings in its own favor with most humans being none the wiser, staging an attack on LA to give reason for its war on smaller A.I.s. Gareth Edwards is lying. He will make another film set in the same universe.
Specifically, a prequel. It will ultimately show how and why the LA bombing happened. May use the "America has been taken over by an AI" WMG above. May be a Stealth Prequel.