Why did FAA ground flights across the US?

BBC News:

Grounding of domestic flights across the US on Wednesday was caused by a relatively obscure system called Notice to Air Missions (Notam), a vital air safety tool.

It is separate from the air traffic control system and warns pilots about hazards along the route.

The system flags everything from flocks of gulls in a flight path to lawnmowers operating near runways.

Pilots must check the system prior to take-off.

The system went down Tuesday night, prompting the US agency responsible for air safety, the Federal Aviation Administration (FAA), to use a telephone hotline as a backup.

But that was quickly overwhelmed as flight traffic picked up on Wednesday morning.

The FAA gave little specific detail about what caused the Notam outage, but the White House said there was no evidence of a cyberattack “at this point”.