Hermione had been responsible for busting the woman's nose. Bellatrix had been dueling Tonks and come close to killing her when Hermione had interrupted the fight and forced Bellatrix back into the great hall. Tonks had called to Hermione to stay out of it but she was so exhausted and weak she had collapsed against the stone steps of the entrance hall panting and trying to recover. Bellatrix had grown tired of playing with Hermione and eventually charged at her using some spell to make her move much faster than was natural.