Er, it may have been on purpose, but more in a blind rage sort of way. From what I remember they were in a very small area with a tightly packed and unruly audience standing very close to the fight. The two of them wheeled very close to the onlookers, and then someone got in the way of the Mountain, which I think prevented him from delivering a potentially critical blow to the viper. The mountain became enraged and pushed the person out of the way, but then other person got in the way right as the viper dodged another one of the mountain's blows, and it sliced right throw the onlooker.
That was the semi-accurate version. I'm sure I missed some detail, but even so, it would have been cool if they included it, because there were a few orderlies and squires in the area who it could have happened to. For instance when the viper's squire was handing him his second spear, he could have swung at him and accidentally struck the squire instead.