Is all barking fear driven?

I am asking this due to the following situation:
Yesterday i took my dog out of the kennel to train obedience and a relative of mine that was at my house appeared in the scenario. She came along with me on the heel and i told her to sit. She sat immediately and then i told the guy(big guy, 2m, 115kg) to approach us. As he was getting near us(he was making noises and faces) she started barking until he stopped or i told her to stop. When i told her to stop she immediately stopped but i didn't feel confident enough to let the guy get very near. She didn't back up or show any signs of avoidance(except the barking, which i don't know how to interpret). The interesting point is that she hasn't been taught this behavior and so i don't know how to interpret this. I would be very glad if someone could enlighten this to me.
By the way, it's a 13mo female purebred german sow line gsd(7/14 are schIII, 3/14 SchII 2/14 schI, the other two are only obedience selected).
If anyone think the pedigree could be useful for something, the link is here: http://www.sbcpa.com.br/Des/Consultas/PedigreeArvore.asp?Reg=SBCPA%20169384

Thanks in advance,

Jose Zullo

If you are walking in the park and you see a man yelling at another man in a forgein language, is he angry or just trying to get his attention? You could probably get an idea based on the tone of voice, but could you be sure?

Dogs use barking for communication. Most of dog communication is non-verbal, so those cues are very important in identifying what the motivation is for the barking.

In evaluating this dog I would first look at the level of socialization the dog has had. If the dog is not well socialized, my guess would be fear. If the dog looks excited to see the person, and seems to want contact with the person I would guess it may come from excitement at meeting some one new. Since the barking stoped when the guy stopped moving towards the dog, I would think fear is a good bet.