Right, so thanks to the cancellations yesterday, they have crew out of position for the night stops. The afternoon departure crew today are nightstopping for tomorrow's flight, and BA didn't think / couldn't find /didn't want an extra crew to fly on that afternoon flight. Ergo crew flies in tonight, both crews swap aircraft, one aircraft stays in WAW and the other comes back heavily delayed. This is airline convenience, EC261 if it goes over 3 hours.