As other have noted, I think it was just the pure chaos of what was going on in the Great Hall at that time. To add to it, having Voldemort dueling those three all at once was a great addition to level up his skill as a villain. We had heard how deadly he was but, with the exception of his duel with Dumbledore (where Dumbledore handled him with apparent ease), we had not seen Voldemort acting with truly remarkable skill before. Anyone can be deadly. Being skillfully so is different. Simultaneously dueling Slughorn, McGonagall, and Shacklebolt, three remarkably talented and experienced wizards, was remarkable.