The best explanation I've heard so far is that they're from the US military who have received intelligence that there may be threats to certain military sites. That would explain a lot of things: 1) Why they won't just shoot one down, or take other measures to find out what they are; 2) Why they're being so tight-lipped about it; 3) Why they're being seen around military sites. But it's still not totally convincing. Don't they have more stealthy ways to monitor these sites? What did they think was going to happen if they sent up drones over populated areas night after night?