At this point, my best guess is that what may have started out as a military drill by the nearby Army base, or perhaps a commercial use for inspection, etc..., once it generated concern, it was halted, and then copycats started piling on in droves, and are getting a good laugh out of it, watching the public panic, and knowing nothing can legally be done about it.
If it's true what authorities are saying that they pose no public risk, then they must know the source, and it could be a great many sources. If they were a threat, I think they would be quickly eliminated by the military. Anyway, it is disturbing, and I think once sources are revealed and laws are modified, hopefully we will see a bit more control.
However, I do think seeing drones in the sky will become more common if drone deliveries continue to expand, and eventually air taxi's become an alternative to avoiding cross-town traffic in crowded cities.
Here's a tidbit on Amazon's drone deliveries taking off:
MSN