Bellatrix found him sleeping with another girl deatheater (who was a half-blood which made it worse) so to get revenge on him for 'cheating' on her she punched him in the face then hexed his nose. However they weren't actually together just Bellatrix was to in love to get the hint from Voldemort so to get rid of her as she was to clingy he staged the scene with the girl to get rid of Bellatrix.

~District_Gryffindor

I hope that makes sense!!