https://www.bbc.co.uk/news/articles/c9d330wqq2zo'safe', for all small and dubious values of 'safe':
"The sound of them crashing on the wharf was considerable even from probably 10 to 15 or 20 metres away; you could hear them physically crash and smash onto the cement marina,"
From the display organiser;
The main cause was "an unforeseen change in the radio frequency environment occurring after take-off," Skymagic said, triggering some drones "to enact failsafe landing procedures in response to compromised positional accuracy".
Someone jammed them? Either directed comms, or GPS?
Perhaps time for "Dumpster Diving - Expert Edition"?