|Fox31 In Denver Reporting United Airlines Computer Failure|
The reason is a software or hardware failure described as "network connectivity." As is usually the case, the carrier doesn't refer to it as "failure."
CNN reported that the problem occurred at "multiple airports," but an on-air reporter at WNYC indicated that local airports may not have extensive delays as of this report (10A July 8, 2015).
UPDATE The problem was rectified about 90 minutes later.
United Airlines System Failure History
In summer of 2011, United Airlines also experienced a system-wide outage. And there were dispatch system outages in November 2012 and again just last month.
Image credit: @KDVR