May economists are predicting ~10% GDP growth for the US this year due to massive pent-up demand. Those who have still been collecting a paycheck or are not dependent on a paycheck have had little to spend their money on for the last year and quite frankly people are sick of sitting at home and want to get out and do things.
I think Delta is just trying to get in front of the curve. I've flown a few times on Delta over the last six months - and the change just from November to January was pretty dramatic - going from maybe 50% of available seats (~1/3rd of the seats being blocked and unavailable) to near 100% of available seats.