From reading a lot of stuff yesterday, they had a significant failure of their crew deployment system. Apparently, flight crews and pilots call a number to get their daily flight assignments and a lot of people ready and able to work, couldn’t do that yesterday. Their linear model vs. hub & spoke relies on quick turnover and so things snowballed quickly given the other issues of tight staffing (true in good/normal conditions), let alone people out with illness, burnout and now this.
The weather problems from earlier, certainly set the board unfavorably, but yesterday was operational failures due to a fragile and outdated technical system combined with insufficient staff for their flight schedule. They lost all their wiggle room in the system for staffing, due to post-pandemic realities but tried to plow ahead anyway. So IMO, this was a predictable outcome. When you need most things to go right for the situation to keep the balls or planes in the air, and the situation is multiple sub-failures, the overall system is going to melt down.