Dog help - blood in image

Was with my dog at the park, he was chasing a ball when another dog may have collided with him. We’re not sure as it was dark out, but the other dog came out severely wounded. I’m not sure how this could have transpired- I checked my dog’s paws, they’re not even remotely sharp and I doubt a tooth could do such damage. We checked the field over and over for sharp objects and found nothing, so the only possible answer is my dog. Could a dull nail have gotten hooked on her pelt? Or could this a result of a bite? She does have a very thin coat. I wasn’t sure where to ask- he’s not an aggressive dog, but he is part German shepherd, so he could have had an innate defensive moment and attacked? He’s best friends with these dogs and gets along well with others but can rough house sometimes, but NEVER something to this extent. Dog went to Emergency Room and had a successful surgery, got stitched up. I’m just trying to figure out the cause, and worried this might happen in the future. I truly hope it was just an accident .

Was with my dog at the park, he was chasing a ball when another dog may have collided with him. We’re not sure as it was dark out, but the other dog came out severely wounded. I’m not sure how this could have transpired- I checked my dog’s paws, they’re not even remotely sharp and I doubt a tooth could do such damage. We checked the field over and over for sharp objects and found nothing, so the only possible answer is my dog. Could a dull nail have gotten hooked on her pelt? Or could this a result of a bite? She does have a very thin coat. I wasn’t sure where to ask- he’s not an aggressive dog, but he is part German shepherd, so he could have had an innate defensive moment and attacked? He’s best friends with these dogs and gets along well with others but can rough house sometimes, but NEVER something to this extent. Dog went to Emergency Room and had a successful surgery, got stitched up. I’m just trying to figure out the cause, and worried this might happen in the future. I truly hope it was just an accident .