As others have already said... you have a border collie, border collies are natural herders. It is hard to take the natural instinct out of the dog and personally why would you? If you don't like the way the dog acts at the dog park then find another outlet for exercise. I also agree it doesn't sound like agression but more natural instinct which does occur more as the dog matures. At 1.5 years you can expect the maturity of your dog and it's natural instincts to kick in full force. Most border collies that I know do not like other animals coming up from behind since all their herding insticts want the dogs in front.

MHO, appreciate the traits of your breed and learn to adapt to them.