"Automation issues" grounded literally every United aircraft this morning before the FAA's Air Traffic Control System Command Center removed the ground stop around 9:45AM Eastern Time. "Automation issues" is another way of saying that United is having trouble with the back-end software and hardware that it uses to coordinate flights at the several hundred airports it serves. Before it lifted, the grounding ended up affecting "mainline" aircraft only, meaning that United partners — the companies that operate regional United Express aircraft — were free to depart.

This isn't the first time a software glitch has grounded United aircraft: an issue in early June resulted in a ground stop lasting roughly 30 minutes, but this one ran longer: all told, flights were stuck at airports for nearly two hours.

We experienced a network connectivity issue. We are working to resolve and apologize for any inconvenience. — United (@united) July 8, 2015

In a tweet, United described today's problem as a "network connectivity issue." Most of the airline's flight operations are based at a command center in Chicago's Willis Tower, but it's unclear whether that facility was the nexus of today's glitch.