Apparently it's another one of those scenarios in which robots have taken over and are now dispatching the weak, fleshy humans (though is this global, merely regional? It isn't explained). It's supposed to be playing off of drones (a scene was actually cut of them being remotely operated by someone), and the particular design is based on something that actually exists in the here and now, as someone posted a few pages back. Things have gotten so bleak, that her and the others risked their lives for a toy to put a smile on a kid's face, a momentary distraction from their existence of trying to survive against the "dogs." But as you say, we have no idea how the current situation came to be. Were they originally security guard dogs and branched out (technology gone out of control and evolved past their original purpose), what?