Turning against his own pack

Question
Last year, there was an interesting story in California. A very young little girl went missing in the woods and a large rescue group (men, women and dogs) set out to look for her.

One shepard dog found the girl and alerted members with his bark. When one of the (human) rescuers showed up, the dog would not let his teammate near the girl. The dog stood between the man and the girl and barked viciously at someone the dog is supposed to be working with.

Every single time the man tried to move towards the girl, the dog would do take on the 'threatening stance (head forward, arched back, hair standing up and eyes wide open) and bark viciously at the man.

For about 20 minutes, the human rescuer had to reassure the dog that he is no threat to the girl before the rest of the team showed up. Ultimately, the dog eased up when the remaining group showed up; they returned the girl to her parents. The shepard and the man have a work relationship before this incident.

Why did the dog act that way with someone who is supposed to be his comrade?  

Answer
Where's Findley when we need him?  This is a puzzle.  I guess it is possible that even if the dog trusted the man, the girl may have been frightened of him.  Unfortunately it is more important than ever to teach children to fear strangers.  Being lost and then having a lone stranger show up may have left the girl panicked. Dogs are quick to read our emotions.  

I put some older pictures up at http://www.photolocker.net/images/Labman/gretchenandellaplaying.jpg

http://www.photolocker.net/images/Labman/labmanellagretchen.jpg  I need to put some ones newer up.  I have a 5 month old Lab/Golden cross now, a sweet little girl.