It was more than likely an accident. I wouldn't worry too much about it. Just keep an eye out in the future. My two beat each other up all the time, lol. My mutt has scratches and scrapes on his face, belly and insides of his legs because of my APBT
pup, and she's got spots of hair missing, scrapes and knots where he tackles her, slings her across the room, and drags her around by the scruff of her neck! I don't let them get too rough with each other, and if they start getting rough, I separate them for about a half an hour.