dogs are predators
Some dogs simply love to chase and bring down prey. They are, after all, closely related to wolves. I love dogs, but not the ones who attack. I have a very gentle dog. As a runner and bicyclist, I have been threatened by a great many dogs. There are hundreds of thousands of emergency room visits resulting from dog attacks every year in the United States. This thing crossed the street from quite a distance. I was not even running at the time - my calf had given me some trouble and I was walking the rest of the distance home. They are even more likely to chase if you are on a bike or running. Perhaps the reason I did not act sooner with this dog was that I had previously been chased by so many that I had gotten used to it. As I said, they usually pull up short, unless you are on a bike and then they like to go for your legs if you let them.