Why is Delta cancelling hundreds of flights?

After a power outage in Atlanta on August 8th, 2016 at 2:30 am, Delta's entire computer system was temporarily down, and even though they started resuming flights just after 8:30, it takes a long time to get the whole system up and running again after that long of a delay. So, flights continued to be cancelled that day (about 1,000) and the next day (about 530), and likely will continue while they try to catch up. They are providing vouchers for people whose flights are cancelled or delayed more than 3 hours.

The problem was extra-severe because Atlanta is one of Delta's main hubs, and most of the cancellations and delays involved that airport.