A better question is what has the owner of the dog done to it. The fact that it's out shows they are at the very least negligent.
Oh it is clear there was something not right with that dog or the situation. We have owned many dogs over the years and have close family that breed bull mastiffs. Every once in a while a born “bad†dog comes along. They are put down at the first sign of aggression to humans around here. I have had to put one down. From a pup on it would not take any discipline action -even verbal with out lashing out. In this case the dog spotted the kid looking under the car and made a bee line for him. Odd.