I only saw the aftermath - it was in a car park and I rounded a car and their they were. My guess is the sparrowhawk tried to grab the pigeon at low level and instead slammed them both into the tarmac. The pigeon had a clear 'WTF just happened?' look, at least until it looked down and decided the chest of a sparrowhawk might not be the best spot for existential debate, at which point it flew off. The sparrowhawk then rolled upright and stalked off behind the nearest car in a huff.