NavList:

But drones have backup modes. Some professional drones will go into a drift mode. Others will automatically land. I would guess that there are different ways of managing big flocks of drones. There are lots of articles online about dealing with GPS dropouts in drones. Here's one. 

There's an interesting quote in the article:

“These are professional drones, which are already built with technologies that would direct them back to the takeoff origin,” Hong Kong Tourism Board’s Executive Director Anthony Lau told the Morning Post, “but the signals were so strong that many of them just dropped from the air.”

That doesn't sound quite right. It suggests that the jamming was also affecting the pilotig systems. 

Frank Reed