Why did my 7 month old Dane bite my 6 yr old niece in the face while eating?

This isthe first time he has done this. My husband, sons (ages 14 and 13), daughter (age 6, same age as my niece) can walk past him and he won't do anything. She says she didn't touch him but I think she may have accidentally touched him. I have always taken his food away while eating. We always thought he knew that anyone could walk by him while eating. This is the first time my niece was near him while eating. He did leave a good mark on her. She doesn't need stitches (thank goodness) but I am very upset over this. I did freak out on him but didn't hit him. He knew right away that he did wrong and went to his cage. I was in the kitchen when it happened so he wasn't unsupervised. I just don't understand and I am still upset.