What prompts Tom to break Myrtle's nose?

There are several contributing factors to the incident of Myrtle's broken nose. Myrtle and Tom are both married to other people and are having an affair. They have to have somewhere else to meet, where they will not be discovered, so Tom has an apartment in the city. When they go there they drink alcohol, frequently a LOT of alcohol. Tom is kind of a bully; he uses his anger to get what he wants and orders other people around. On the day he breaks Myrtle's nose, the two of them and several others have gone to the apartment. They are all drinking. Myrtle starts saying Daisy's name. Tom tells her to stop, but she keeps chanting Daisy's name until Tom backhands her across the face, breaking her nose.