What's his problem?

My neighbor across the street has two dogs, one of them Ginger absolutely adores even though the dog is indifferent to her.

The other one is younger, maybe two. He barks warmly and wags his tail (in a friendly way, I can tell) and assumes the inviting play position with his bottom up and his head towards the ground.

Anyways, my baby being the sweetie that she is went over to say hi one day and all of a sudden the dog attacked her! He went from being friendly one second to being a jerk in the next.

I work with dogs and know a lot about them but don't understand it. What could be the reason behind this? Ginger has gone over there before with no problems.

Anyways, Ginger can't see her friend anymore because of the other dog. I feel really bad for her.