Older dogs can definately develop dementia just like people do. There could some confusion happening in his brain due to his age. He may well not have known what/who he was biting. I'd take him back to the vet and just get him checked over. I'd have his eyes checked as well. When Preacher first started to loose his sight, we saw an increase in aggression from him. He didn't mean to do it, he was just freaked out because he could no longer see what was coming at him.

I've been nailed a couple times by my own dogs. It's always stressful, even when I could explain why it happened--once I was pulling Kayleigh off another dog, once I reached in to take a treat from Preacher and forgot he couldn't see anymore.