Computers fail. Entire server farms full of computers fail. Nothing electronic is failsafe. So, the full stop that grounded United Airlines on the morning of July 8, 2015 might simply be a tech problem. Such issues have occurred before and will again.
It is worth it to note, however, that United Airlines has offered rewards to hackers who find security flaws in the company’s systems. Those rewards could be considered a challenge, or they could be considered by some an implied admission that there are flaws to find.
And as Sophos’s Naked Security blog also notes, United has had issues in the past with data insecurity.