Yes it's an instinctual thing. Doggies just love to chase - cars, bikes, whatever. They are also protecting their territory. You know, all those places they've hiked up a leg? If need be hop off the bike and hold it between you and the naughty pooch and use the front and rear wheels as weapons swinging them violently back and forth. Yelling works too. I have yet to beat on my chest like a great silverback but whatever it takes to out alpha male the alpha male I suppose.
I just realized that my territory is very limited to the bog and a little smattering about on the floor!