Aussie bit someone on the ear

Question
Hello! We adopted an aussie mix (don't know what the mix is) from the Humane Society a few months ago.  He is very gentle and cuddly with us, though he has become more protective the last month or so.  The vet guesses he is about 4 years old.  Though we do not know his past, he seems to be comfortable around other dogs and people.  Everyday we either take him to the dog park (many dogs and owners) or for walks in a busy public park and he always does very well with dogs of all sizes, children and adults.
Yesterday our 7 year old neighbor came over to play.  We were on the floor playing a game and the child jumped up to grab a movie off the shelf in another part of the room.  Our aussie jumped up and bit the cartilage of her ear.  He immediately got off the child (we didn't have to pull him off her) and didn't appear to be aggressive.  We can't figure out why he bit her!  Was this a herding extinct?  Is there anyway to prevent this in the future? Thank you for your help!

Answer
The Australian Shepherd breed standard says that they are a dog with "strong herding and guarding instinct".  Herding instinct basically means that they have an inherent desire to stop motion.  That's why they are seldom recommended for families with younger children who cannot remain still, and may shriek or run or make quick movements.  The movement is what usually engages the dog's predatory drive (in herding dogs, predatory drive is very strong, but in most of them the "kill instinct" has been bred out).  Aussies love their families, but some of them are not going to be everyone else's friend.  Away from home, many of them are still reserved with strangers, or guardy of their humans, but others, like yours, are fine.  This is a dog that I would crate when guests come, especially children, and not take chances.  (If the intended target keeps moving, some of them go back again for a quick nip - herders generally don't attach themselves and hang on - they are more likely to grab and drop, frequently from behind, as they would when driving livestock).  You could acclimate him to a muzzle and have him wear it when you have company, but don't think you will make him everyone's best bud, because chances are you won't.  Of course, properly managed, he can still be a good dog for your family, but you have to do that.  To learn to acclimate to a muzzle, check out the Morrco.com site.
And, if you want to learn more about the Aussie breed, Google on Pincie Creek Aussies - there's a good explanation under "is an Aussie for you" section.