While I found the previous "CIA drones" answer amusing, I'd go with either someone not all there or someone trying to catch a meal. And when these things start showing up, I think it tends to be a "oh, check this video - I saw something like that, too" and they go back and take video, or already have it and decide to post it. Someone sees a thing, other people realize they've seen a similar thing, and then you get a rash of postings. Kind of the way cat or talking husky videos got started.
If you pay attention to what's going on, I'm sure you could find homeless folks around city parks or in alleys trying to catch pigeons or other birds, "rat fishing", etc., just because they need a meal and either don't want to go to the nearby shelter, got there late, don't feel like dumpster diving, etc. Sadly, there are some desperate people around and you can likely see it happening every day if you live in an area with a lot of homeless and make the effort to look. Humans don't like to see it, though, so we often don't, even if it is all around us. It's the human condition and it isn't always pretty.