Unknown to be of hardware or software, or people error origin, via a quoted Katie Connel.

Delta Air had a computer problem, on Saturday. Cancellation of 40 or so flights, came about. Also an unknown number of flight delays were caused.

Source
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"Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!"