Answer: Drone sightings.
Gatwick Airport in London put everything on lockdown Thursday morning, including all outbound and inbound flights. The reason: drones.
Airport authorities grounded all flights when they heard reports of two drones flying near the airport on Wednesday night. The Sussex police department was called in to assist in an investigation, but no further details were released. Airport staff were doing their best to accommodate affected passengers, but as of midday Thursday, all flights were still grounded.
As the number of privately owned drones has increased, so too have concerns about the safety hazards that they create for airplanes. During the California Wine Country Fires in 2017, planes needed for suppression efforts were grounded when privately owned drones were spotted flying near the fires.