Certainly anecdotal, but my only canceled flight last year was on WN from SAN-RNO. 91 segments on DL-0 cancellations, 6 segments on WN. No weather issues really anywhere in the western US at that time. They cancelled it the night before, about 12 hours ahead of schedule departure. My quess was they needed the plane elsewhere and our flight had the lightest load, but I wasn't too pleased. I only take WN when they offer a nonstop flight that I can't get else where, so I certainly wasn't going to take any of their significantly longer rerouting options.