From the article:
“Reuters reports that last week’s computer glitch at a California air traffic control center that led officials to halt takeoffs at Los Angeles International Airport was caused by a U-2 spy plane still in use by the US military, passing through air space monitored by the Los Angeles Air Route Traffic Control Center that appears to have overloaded ERAM, a computer system at the center. According to NBC News, computers at the center began operations to prevent the U-2 from colliding with other aircraft, even though the U-2 was flying at an altitude of 60,000 feet and other airplanes passing through the region’s air space were miles below.”
One of the commenters on the post suggested the aircraft was flying VFR-on-Top, a set of rules governing how the ATC systems respond to potential collisions in airspace and warn aircraft (or operators) to move. The system was only designed to respond effectively to aircraft using this specification below 18,000 feet, but the U-2 was flying at 60,000 ft. Essentially every aircraft below that ceiling was “in the way” and the system overloaded attempting to parcel out commands to move those aircraft out the way.