Here's Chick-fil-A CEO Dan Cathy's full statement about how it all unfolded, as reported by WHIO:
“The mayor called about 10 p.m. and asked for assistance. We immediately mobilized staff and team members who live and work near the airport, and they are making sandwiches and delivering them to the EOC (emergency operations center). City and airport officials there are distributing sandwiches to passengers who are stranded due to the power outage. It has been a very difficult day for thousands of travelers, and while Chick-fil-A is always closed on Sunday, our restaurants open occasionally to serve communities in need. We do not make a profit, but do what we can to offer comfort to people experiencing hardship”
As to why Chick-fil-A is closed on Sundays -- basically it's to give workers a day to spend with friends and family.
The Atlanta airport is the world's busiest, so any day without power there is a disaster. This may not have been a Christmas miracle, but it was certainly a nice thing.