The scene seems to have been set when one senior ATi mischief maker (who, like one of its previous marketing directors the night before), probably shouldn't have really been anywhere near the NVIDIA party) went out of his way to try and engage with an NVIDIA girl in more than just idle and polite chatter.
When his opposite number at NVIDIA seemed to decide that there may be some strategic motives against NVIDIA'S interests behind the situation, matters became increasingly tense, and NVIDIA people followed ATi staff outside the Munchner Halle, and its reported that both camps effectively squared up to each other.